[msl] Polyfill signed integer arithmetic

Avoid UB caused by signed integer overflow by bitcasting to unsigned
integers and back again.

Bug: 42251016
Change-Id: I98078a9a4e9603b3ab6faa4dbc0fc0812a5dcc3e
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/205235
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Commit-Queue: James Price <jrprice@google.com>

test/tint/array/assign_to_subexpr.wgsl.expected.ir.msl

diff --git a/test/tint/array/assign_to_subexpr.wgsl.expected.ir.msl b/test/tint/array/assign_to_subexpr.wgsl.expected.ir.msl
index 265bd44..c7b0190 100644
--- a/test/tint/array/assign_to_subexpr.wgsl.expected.ir.msl
+++ b/test/tint/array/assign_to_subexpr.wgsl.expected.ir.msl
@@ -34,7 +34,7 @@
   (*dst_ptr) = src;
   (*dst_struct_ptr).arr = src;
   (*dst_array_ptr)[0] = src;
-  return (((*dst_ptr)[0] + (*dst_struct_ptr).arr[0]) + (*dst_array_ptr)[0][0]);
+  return as_type<int>((as_type<uint>(as_type<int>((as_type<uint>((*dst_ptr)[0]) + as_type<uint>((*dst_struct_ptr).arr[0])))) + as_type<uint>((*dst_array_ptr)[0][0])));
 }
 
 kernel void tint_symbol(device int* s [[buffer(0)]]) {

test/tint/array/type_initializer.wgsl.expected.ir.msl

diff --git a/test/tint/array/type_initializer.wgsl.expected.ir.msl b/test/tint/array/type_initializer.wgsl.expected.ir.msl
index 1f0ec3b..bebcd67 100644
--- a/test/tint/array/type_initializer.wgsl.expected.ir.msl
+++ b/test/tint/array/type_initializer.wgsl.expected.ir.msl
@@ -22,16 +22,16 @@
   int const x = 42;
   tint_array<int, 4> const empty = tint_array<int, 4>{};
   tint_array<int, 4> const nonempty = tint_array<int, 4>{1, 2, 3, 4};
-  tint_array<int, 4> const nonempty_with_expr = tint_array<int, 4>{1, x, (x + 1), nonempty[3]};
+  tint_array<int, 4> const nonempty_with_expr = tint_array<int, 4>{1, x, as_type<int>((as_type<uint>(x) + as_type<uint>(1))), nonempty[3]};
   tint_array<tint_array<tint_array<int, 4>, 3>, 2> const nested_empty = tint_array<tint_array<tint_array<int, 4>, 3>, 2>{};
   tint_array<tint_array<tint_array<int, 4>, 3>, 2> const nested_nonempty = tint_array<tint_array<tint_array<int, 4>, 3>, 2>{tint_array<tint_array<int, 4>, 3>{tint_array<int, 4>{1, 2, 3, 4}, tint_array<int, 4>{5, 6, 7, 8}, tint_array<int, 4>{9, 10, 11, 12}}, tint_array<tint_array<int, 4>, 3>{tint_array<int, 4>{13, 14, 15, 16}, tint_array<int, 4>{17, 18, 19, 20}, tint_array<int, 4>{21, 22, 23, 24}}};
-  tint_array<int, 4> const v = tint_array<int, 4>{1, 2, x, (x + 1)};
-  tint_array<tint_array<tint_array<int, 4>, 3>, 2> const nested_nonempty_with_expr = tint_array<tint_array<tint_array<int, 4>, 3>, 2>{tint_array<tint_array<int, 4>, 3>{v, tint_array<int, 4>{5, 6, nonempty[2], (nonempty[3] + 1)}, nonempty}, nested_nonempty[1]};
+  tint_array<int, 4> const v = tint_array<int, 4>{1, 2, x, as_type<int>((as_type<uint>(x) + as_type<uint>(1)))};
+  tint_array<tint_array<tint_array<int, 4>, 3>, 2> const nested_nonempty_with_expr = tint_array<tint_array<tint_array<int, 4>, 3>, 2>{tint_array<tint_array<int, 4>, 3>{v, tint_array<int, 4>{5, 6, nonempty[2], as_type<int>((as_type<uint>(nonempty[3]) + as_type<uint>(1)))}, nonempty}, nested_nonempty[1]};
   int const subexpr_empty = 0;
   int const subexpr_nonempty = 3;
-  int const subexpr_nonempty_with_expr = tint_array<int, 4>{1, x, (x + 1), nonempty[3]}[2];
+  int const subexpr_nonempty_with_expr = tint_array<int, 4>{1, x, as_type<int>((as_type<uint>(x) + as_type<uint>(1))), nonempty[3]}[2];
   tint_array<int, 4> const subexpr_nested_empty = tint_array<int, 4>{};
   tint_array<int, 4> const subexpr_nested_nonempty = tint_array<int, 4>{5, 6, 7, 8};
-  tint_array<int, 4> const subexpr_nested_nonempty_with_expr = tint_array<tint_array<int, 4>, 2>{tint_array<int, 4>{1, x, (x + 1), nonempty[3]}, nested_nonempty[1][2]}[1];
-  (*tint_module_vars.s) = (((((((((((empty[0] + nonempty[0]) + nonempty_with_expr[0]) + nested_empty[0][0][0]) + nested_nonempty[0][0][0]) + nested_nonempty_with_expr[0][0][0]) + subexpr_empty) + subexpr_nonempty) + subexpr_nonempty_with_expr) + subexpr_nested_empty[0]) + subexpr_nested_nonempty[0]) + subexpr_nested_nonempty_with_expr[0]);
+  tint_array<int, 4> const subexpr_nested_nonempty_with_expr = tint_array<tint_array<int, 4>, 2>{tint_array<int, 4>{1, x, as_type<int>((as_type<uint>(x) + as_type<uint>(1))), nonempty[3]}, nested_nonempty[1][2]}[1];
+  (*tint_module_vars.s) = as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(as_type<int>((as_type<uint>(empty[0]) + as_type<uint>(nonempty[0])))) + as_type<uint>(nonempty_with_expr[0])))) + as_type<uint>(nested_empty[0][0][0])))) + as_type<uint>(nested_nonempty[0][0][0])))) + as_type<uint>(nested_nonempty_with_expr[0][0][0])))) + as_type<uint>(subexpr_empty)))) + as_type<uint>(subexpr_nonempty)))) + as_type<uint>(subexpr_nonempty_with_expr)))) + as_type<uint>(subexpr_nested_empty[0])))) + as_type<uint>(subexpr_nested_nonempty[0])))) + as_type<uint>(subexpr_nested_nonempty_with_expr[0])));
 }