i965: Generalize the pixel_x/y workaround for all UW types.

i965: Generalize the pixel_x/y workaround for all UW types.

This is the only case where a fs_reg in brw_fs_visitor is used during
optimization/code generation, and it meant that optimizations had to be
careful to not move pixel_x/y's register number without updating it.

Additionally, it turns out we had a couple of other UW values that weren't
getting this treatment (like gl_SampleID), so this more general fix is
probably a good idea (though I wasn't able to replicate problems with
either pixel_[xy]'s values or gl_SampleID, even when telling the register
allocator to reuse registers immediately)

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>