Revert "Revert "glsl: skip stringification in preprocessor if in unreachable branch""

This reverts commit 2fd6f06f.

Take back 28a3731e ("glsl: skip stringification in preprocessor if in
unreachable branch") after b9fe8ff2 ("glsl: fixer lexer for
unreachable defines") has made it to the branch.

Signed-off-by: Andres Gomez <agomez@igalia.com>