Implement FXC workaround for vector access in loops resulting in failed loop unrolling

When indexing into vectors in a loop, FXC sometimes fails to determine
the max number of iterations when attempting to unroll the loop,
resulting in "error X3511: forced to unroll loop, but unrolling
failed.". We work around this by calling a function that sets the input
value at the input index into an inout vector. This seems to nudge FXC
enough for it to determine the number of loop iterations to unroll.

Bug: tint:534
Change-Id: I52cb209be29fcad8fbb91283c7be8c6e22e00656
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56140
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

test/bug/tint/534.wgsl

@@ -0,0 +1,48 @@
+[[block]] struct Uniforms {
+    dstTextureFlipY : u32;
+    isFloat16 : u32;
+    isRGB10A2Unorm : u32;
+    channelCount : u32;
+};
+[[block]] struct OutputBuf {
+    result : [[stride(4)]] array<u32>;
+};
+[[group(0), binding(0)]] var src : texture_2d<f32>;
+[[group(0), binding(1)]] var dst : texture_2d<f32>;
+[[group(0), binding(2)]] var<storage_buffer, read_write> output : OutputBuf;
+[[group(0), binding(3)]] var<uniform> uniforms : Uniforms;
+//[[builtin(global_invocation_id)]] var<in> GlobalInvocationID : vec3<u32>;
+// Fp16 logic
+// Infinity and NaN won't happen in this test case.
+fn ConvertToFp16FloatValue(fp32 : f32) -> u32 {
+    return 1u;
+}
+
+[[stage(compute), workgroup_size(1, 1, 1)]]
+fn main([[builtin(global_invocation_id)]] GlobalInvocationID : vec3<u32>) {
+    var size : vec2<i32> = textureDimensions(src);
+    var dstTexCoord : vec2<i32> = vec2<i32>(GlobalInvocationID.xy);
+    var srcTexCoord : vec2<i32> = dstTexCoord;
+    if (uniforms.dstTextureFlipY == 1u) {
+        srcTexCoord.y = size.y - dstTexCoord.y - 1;
+    }
+
+    var srcColor : vec4<f32> = textureLoad(src, srcTexCoord, 0);
+    var dstColor : vec4<f32> = textureLoad(dst, dstTexCoord, 0);
+    var success : bool = true;
+
+    var srcColorBits : vec4<u32>;
+    var dstColorBits : vec4<u32> = vec4<u32>(dstColor);
+
+    for (var i : u32 = 0u; i < uniforms.channelCount; i = i + 1u) {
+            srcColorBits[i] = ConvertToFp16FloatValue(srcColor[i]);
+            success = success && (srcColorBits[i] == dstColorBits[i]);
+    }
+
+    var outputIndex : u32 = GlobalInvocationID.y * u32(size.x) + GlobalInvocationID.x;
+    if (success) {
+        output.result[outputIndex] = u32(1);
+    } else {
+        output.result[outputIndex] = u32(0);
+    }
+}