glsl: skip stringification in preprocessor if in unreachable branch

This fixes compilation of some "No Mans Sky" shaders where the stringification
happens in branches intended for DX12.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/glsl/glcpp/glcpp-lex.l

@@ -420,8 +420,10 @@ HEXADECIMAL_INTEGER	0[xX][0-9a-fA-F]+[uU]?
 
 	/* This will catch any non-directive garbage after a HASH */
 <HASH>{NONSPACE} {
-	BEGIN INITIAL;
-	RETURN_TOKEN (GARBAGE);
+	if (!parser->skipping) {
+		BEGIN INITIAL;
+		RETURN_TOKEN (GARBAGE);
+	}
 }
 
 	/* An identifier immediately followed by '(' */