tint/val: Make DXC validation output stable

Replace the temporary file name with 'shader.hlsl', so that
skip-expectations can be stably re-generated.

Change-Id: I5ead2235e6e0d84ad67c8d90f8d06b812c8fd593
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/97145
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
diff --git a/test/tint/unittest/reader/spirv/SpvParserFunctionVarTest_EmitStatement_Phi_FromHeaderAndThen.spvasm.expected.dxc.hlsl b/test/tint/unittest/reader/spirv/SpvParserFunctionVarTest_EmitStatement_Phi_FromHeaderAndThen.spvasm.expected.dxc.hlsl
index 671644c..42a507b 100644
--- a/test/tint/unittest/reader/spirv/SpvParserFunctionVarTest_EmitStatement_Phi_FromHeaderAndThen.spvasm.expected.dxc.hlsl
+++ b/test/tint/unittest/reader/spirv/SpvParserFunctionVarTest_EmitStatement_Phi_FromHeaderAndThen.spvasm.expected.dxc.hlsl
@@ -1,6 +1,5 @@
 SKIP: FAILED
 
-warning: code is unreachable
 static uint x_1 = 0u;
 static bool x_7 = false;
 static bool x_8 = false;
@@ -26,7 +25,6 @@
       }
       continue;
     }
-    return;
     {
       x_1 = x_2_phi;
     }
@@ -38,10 +36,11 @@
   main_1();
   return;
 }
+DXC validation failure:
 warning: DXIL.dll not found.  Resulting DXIL will not be signed for use in release environments.
 
 error: validation errors
-/tmp/tint_nu4hkX:34: error: Loop must have break.
+shader.hlsl:33: error: Loop must have break.
 Validation failed.