[hlsl] Emit `numthreads` from HLSL IR backend.

This CL adds the `numthreads` annotations to the HLSL IR backend compute
shaders.

Bug: 42251045
Change-Id: Icf9ad477c56a3d3f928cf18bb021dd305e07d936
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/193741
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: James Price <jrprice@google.com>
diff --git a/test/tint/expressions/bitcast/const/96bit/vec3i32-vec3i32.wgsl.expected.ir.fxc.hlsl b/test/tint/expressions/bitcast/const/96bit/vec3i32-vec3i32.wgsl.expected.ir.fxc.hlsl
index a5351d1..26abf69 100644
--- a/test/tint/expressions/bitcast/const/96bit/vec3i32-vec3i32.wgsl.expected.ir.fxc.hlsl
+++ b/test/tint/expressions/bitcast/const/96bit/vec3i32-vec3i32.wgsl.expected.ir.fxc.hlsl
@@ -1,3 +1,4 @@
+[numthreads(1, 1, 1)]
 void f() {
   int3 b = int3(1073757184, -1006616064, -998242304);
 }