tint/writer/spirv: Fix build on MSVC / some clang builds

For some reason, only a handful of compilers complain about the implicit cast of an enum to a uint32_t.
Make the cast explicit to fix these builds.

Change-Id: Ib57026cbbb28e330055a5c64eaf1ae05ea3ff7dd
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/131744
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: James Price <jrprice@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
diff --git a/src/tint/writer/spirv/generator_impl_ir.cc b/src/tint/writer/spirv/generator_impl_ir.cc
index e947e6c..855662a 100644
--- a/src/tint/writer/spirv/generator_impl_ir.cc
+++ b/src/tint/writer/spirv/generator_impl_ir.cc
@@ -106,8 +106,9 @@
     });
 
     // Declare the function.
-    auto decl = Instruction{spv::Op::OpFunction,
-                            {return_type_id, id, SpvFunctionControlMaskNone, function_type_id}};
+    auto decl =
+        Instruction{spv::Op::OpFunction,
+                    {return_type_id, id, U32Operand(SpvFunctionControlMaskNone), function_type_id}};
 
     // Create a function that we will add instructions to.
     // TODO(jrprice): Add the parameter declarations when they are supported in the IR.