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/array.wgsl

@@ -1,5 +1,5 @@
 [[stage(compute), workgroup_size(1)]]
 fn main() {
     var v : array<i32, 3>;
-    ignore(v);
+    _ = v;
 }