test/tint/builtins/gen: Use returned value

It appears that FXC and DXC do some validation post dead-code-elimination.
These tests have been updated so that the return value is assigned to a storage buffer, ensuring that all validation is performed.

Many DXC tests are affected by https://github.com/microsoft/DirectXShaderCompiler/issues/5082, which have been SKIP'ed.

Fixed: tint:1859
Change-Id: I0001a9a9821846cd0855c3d8ce2bec79ab8e64ef
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/122662
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>
Commit-Queue: James Price <jrprice@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
diff --git a/test/tint/builtins/gen/literal/smoothstep/6c4975.wgsl.expected.fxc.hlsl b/test/tint/builtins/gen/literal/smoothstep/6c4975.wgsl.expected.fxc.hlsl
index 76df66c..758589d 100644
--- a/test/tint/builtins/gen/literal/smoothstep/6c4975.wgsl.expected.fxc.hlsl
+++ b/test/tint/builtins/gen/literal/smoothstep/6c4975.wgsl.expected.fxc.hlsl
@@ -1,5 +1,8 @@
+RWByteAddressBuffer prevent_dce : register(u0, space2);
+
 void smoothstep_6c4975() {
   float res = 0.5f;
+  prevent_dce.Store(0u, asuint(res));
 }
 
 struct tint_symbol {