tint/test-runner: Split expectations for FXC and DXC

Change tint's `--fxc` flag to take the path of the FXC compiler DLL.
Have tint attempt to validate with both FXC and DXC if `--validate` is
passed.

Fix the 'dirsWithNoPassExpectations' logic which looks like it got
broken with the tint -> dawn merge. It also incorrectly applied
filepath.FromSlash() on windows.

Change-Id: I0f46aa5c21bc48a2abc48402c41f846aff4a8633
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/96800
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@chromium.org>

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

@@ -0,0 +1,60 @@
+bug/fxc/gradient_in_varying_loop/1112.wgsl:23:33 warning: 'textureSample' must only be called from uniform control flow
+        let sampleDepth : f32 = textureSample(depthTexture, Sampler, offset.xy).r;
+                                ^^^^^^^^^^^^^
+
+bug/fxc/gradient_in_varying_loop/1112.wgsl:18:28 note: control flow depends on non-uniform value
+        if (offset.x < 0.0 || offset.y < 0.0 || offset.x > 1.0 || offset.y > 1.0) {
+                           ^^
+
+bug/fxc/gradient_in_varying_loop/1112.wgsl:8:29 note: return value of 'textureSample' may be non-uniform
+    let random: vec3<f32> = textureSample(randomTexture, Sampler, vUV).rgb;
+                            ^^^^^^^^^^^^^
+
+SamplerState tint_symbol : register(s0, space0);
+Texture2D<float4> randomTexture : register(t1, space0);
+Texture2D<float4> depthTexture : register(t2, space0);
+
+struct tint_symbol_2 {
+  float2 vUV : TEXCOORD0;
+};
+struct tint_symbol_3 {
+  float4 value : SV_Target0;
+};
+
+float4 main_inner(float2 vUV) {
+  const float3 random = randomTexture.Sample(tint_symbol, vUV).rgb;
+  int i = 0;
+  [loop] while (true) {
+    if ((i < 1)) {
+    } else {
+      break;
+    }
+    const float3 offset = float3((random.x).xxx);
+    bool tint_tmp_2 = (offset.x < 0.0f);
+    if (!tint_tmp_2) {
+      tint_tmp_2 = (offset.y < 0.0f);
+    }
+    bool tint_tmp_1 = (tint_tmp_2);
+    if (!tint_tmp_1) {
+      tint_tmp_1 = (offset.x > 1.0f);
+    }
+    bool tint_tmp = (tint_tmp_1);
+    if (!tint_tmp) {
+      tint_tmp = (offset.y > 1.0f);
+    }
+    if ((tint_tmp)) {
+      i = (i + 1);
+      continue;
+    }
+    const float sampleDepth = depthTexture.Sample(tint_symbol, offset.xy).r;
+    i = (i + 1);
+  }
+  return (1.0f).xxxx;
+}
+
+tint_symbol_3 main(tint_symbol_2 tint_symbol_1) {
+  const float4 inner_result = main_inner(tint_symbol_1.vUV);
+  tint_symbol_3 wrapper_result = (tint_symbol_3)0;
+  wrapper_result.value = inner_result;
+  return wrapper_result;
+}