test: Update expected output of FXC bug

Looks like this should have been part of bc5d8f6b

Bug: tint:1112
Change-Id: I62e98bbb5f8ca29a171d5d5baa65af55b3a090d4
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/70664
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/bug/fxc/gradient_in_varying_loop/1112.wgsl.expected.hlsl

@@ -12,7 +12,7 @@ struct tint_symbol_3 {
 float4 main_inner(float2 vUV) {
   const float3 random = randomTexture.Sample(tint_symbol, vUV).rgb;
   int i = 0;
-  [loop] while (true) {
+  while (true) {
     if ((i < 1)) {
     } else {
       break;