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/expressions/splat/expression/bool.wgsl.expected.fxc.hlsl

@@ -0,0 +1,22 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  bool tint_tmp = true;
+  if (!tint_tmp) {
+    tint_tmp = false;
+  }
+  bool2 v2 = bool2(((tint_tmp)).xx);
+  bool tint_tmp_1 = true;
+  if (!tint_tmp_1) {
+    tint_tmp_1 = false;
+  }
+  bool3 v3 = bool3(((tint_tmp_1)).xxx);
+  bool tint_tmp_2 = true;
+  if (!tint_tmp_2) {
+    tint_tmp_2 = false;
+  }
+  bool4 v4 = bool4(((tint_tmp_2)).xxxx);
+}

test/tint/expressions/splat/expression/f32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,10 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  float2 v2 = float2(((1.0f + 2.0f)).xx);
+  float3 v3 = float3(((1.0f + 2.0f)).xxx);
+  float4 v4 = float4(((1.0f + 2.0f)).xxxx);
+}

test/tint/expressions/splat/expression/i32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,10 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  int2 v2 = int2(((1 + 2)).xx);
+  int3 v3 = int3(((1 + 2)).xxx);
+  int4 v4 = int4(((1 + 2)).xxxx);
+}

test/tint/expressions/splat/expression/u32.wgsl.expected.fxc.hlsl

@@ -0,0 +1,10 @@
+[numthreads(1, 1, 1)]
+void unused_entry_point() {
+  return;
+}
+
+void f() {
+  uint2 v2 = uint2(((1u + 2u)).xx);
+  uint3 v3 = uint3(((1u + 2u)).xxx);
+  uint4 v4 = uint4(((1u + 2u)).xxxx);
+}