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/builtins/extractBits/vec3/i32.spvasm.expected.fxc.hlsl

@@ -0,0 +1,21 @@
+int3 tint_extract_bits(int3 v, uint offset, uint count) {
+  const uint s = min(offset, 32u);
+  const uint e = min(32u, (s + count));
+  const uint shl = (32u - e);
+  const uint shr = (shl + s);
+  return ((v << uint3((shl).xxx)) >> uint3((shr).xxx));
+}
+
+void f_1() {
+  int3 v = (0).xxx;
+  uint offset_1 = 0u;
+  uint count = 0u;
+  const int3 x_15 = tint_extract_bits(v, offset_1, count);
+  return;
+}
+
+[numthreads(1, 1, 1)]
+void f() {
+  f_1();
+  return;
+}

test/tint/builtins/extractBits/vec3/u32.spvasm.expected.fxc.hlsl

@@ -0,0 +1,21 @@
+uint3 tint_extract_bits(uint3 v, uint offset, uint count) {
+  const uint s = min(offset, 32u);
+  const uint e = min(32u, (s + count));
+  const uint shl = (32u - e);
+  const uint shr = (shl + s);
+  return ((v << uint3((shl).xxx)) >> uint3((shr).xxx));
+}
+
+void f_1() {
+  uint3 v = (0u).xxx;
+  uint offset_1 = 0u;
+  uint count = 0u;
+  const uint3 x_14 = tint_extract_bits(v, offset_1, count);
+  return;
+}
+
+[numthreads(1, 1, 1)]
+void f() {
+  f_1();
+  return;
+}