tint: const eval of length builtin

Bug: tint:1581
Change-Id: Ie6dc9da6b48c606af03da023c835ec36a99dd362
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/110981
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
diff --git a/test/tint/builtins/gen/literal/length/afde8b.wgsl.expected.msl b/test/tint/builtins/gen/literal/length/afde8b.wgsl.expected.msl
index 74668c5..5fed668 100644
--- a/test/tint/builtins/gen/literal/length/afde8b.wgsl.expected.msl
+++ b/test/tint/builtins/gen/literal/length/afde8b.wgsl.expected.msl
@@ -2,7 +2,7 @@
 
 using namespace metal;
 void length_afde8b() {
-  float res = length(float2(1.0f));
+  float res = 0.0f;
 }
 
 struct tint_symbol {