Migrate more tests from Ignore() to phony-assignment

The `Ignore()` intrinsic is about to be deprecated, so don't use it for testing.

Bug: tint:1213
Change-Id: I314ecaeb9a9c337c7b6980189054120a74807ebd
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/67066
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>
Reviewed-by: James Price <jrprice@google.com>

test/var/initialization/function/struct.wgsl.expected.hlsl

@@ -6,6 +6,5 @@ struct S {
 [numthreads(1, 1, 1)]
 void main() {
   S v = (S)0;
-  v;
   return;
 }