intel/fs: Bail in optimize_extract_to_float if we have modifiers

intel/fs: Bail in optimize_extract_to_float if we have modifiers

This fixes a bug in runscape where we were optimizing x >> 16 to an
extract and then negating and converting to float.  The NIR to fs pass
was dropping the negate on the floor breaking a geometry shader and
causing it to render nothing.

Fixes: 1f862e923cb "i965/fs: Optimize float conversions of byte/word..."
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109601
Tested-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>