wgsl: Migrate from ignore() to phony-assignment

The rules have been relaxed about function values always needing to be
consumed, so a bunch of these phony-assignments can be replaced with the
RHS, but this will be done as a follow-up.

Bug: tint:1213
Change-Id: Ie7c4280f87b4ad7e5a429994b0b88ac22c2f3a9f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/67300
Commit-Queue: Ben Clayton <bclayton@google.com>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/tests/white_box/D3D12DescriptorHeapTests.cpp b/src/tests/white_box/D3D12DescriptorHeapTests.cpp
index b38eb5f..4050667 100644
--- a/src/tests/white_box/D3D12DescriptorHeapTests.cpp
+++ b/src/tests/white_box/D3D12DescriptorHeapTests.cpp
@@ -183,7 +183,7 @@
     renderPipelineDescriptor.cFragment.module = utils::CreateShaderModule(device, R"(
             [[group(0), binding(0)]] var sampler0 : sampler;
             [[stage(fragment)]] fn main() -> [[location(0)]] vec4<f32> {
-                ignore(sampler0);
+                _ = sampler0;
                 return vec4<f32>(0.0, 0.0, 0.0, 0.0);
             })");