[msl-writer] Add emission of if statements.

This Cl adds emission of `if`, `else if` and `else` statements to the
MSL backend.

Bug: tint:8
Change-Id: I8c22d70f2afa0a1d86cf475f5c98127504a6dc0e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/23840
Reviewed-by: David Neto <dneto@google.com>

src/CMakeLists.txt

diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index e1cd7a4..27d1bfe 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -472,7 +472,6 @@
     writer/wgsl/generator_impl_cast_test.cc
     writer/wgsl/generator_impl_constructor_test.cc
     writer/wgsl/generator_impl_continue_test.cc
-    writer/wgsl/generator_impl_else_test.cc
     writer/wgsl/generator_impl_entry_point_test.cc
     writer/wgsl/generator_impl_fallthrough_test.cc
     writer/wgsl/generator_impl_function_test.cc
@@ -500,6 +499,7 @@
     writer/msl/generator_impl_constructor_test.cc
     writer/msl/generator_impl_function_test.cc
     writer/msl/generator_impl_identifier_test.cc
+    writer/msl/generator_impl_if_test.cc
     writer/msl/generator_impl_return_test.cc
     writer/msl/generator_impl_test.cc
     writer/msl/generator_impl_type_test.cc