SpirvShader: Mark GLSLstd450IMix as unreachable.

Was removed from the spec in version 0.99.

Bug: b/126873455
Change-Id: I93c60b7d1e8332e70f7116636384f4f85ea542cb
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28708
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
diff --git a/src/Pipeline/SpirvShader.cpp b/src/Pipeline/SpirvShader.cpp
index 797ef73..0f5dc78 100644
--- a/src/Pipeline/SpirvShader.cpp
+++ b/src/Pipeline/SpirvShader.cpp
@@ -3494,7 +3494,7 @@
 		}
 		case GLSLstd450IMix:
 		{
-			UNIMPLEMENTED("GLSLstd450IMix");
+			UNREACHABLE("GLSLstd450IMix has been removed from the specification");
 			break;
 		}
 		case GLSLstd450PackDouble2x32: