intel/fs: Return consistent UW types from sample_mask_reg() in fragment shaders.

intel/fs: Return consistent UW types from sample_mask_reg() in fragment shaders.

In SIMD32 programs that don't use discard, the upper 16 bits of the UD
result of sample_mask_reg() don't contain the sample mask of the upper
16 channels as one would expect.  Stop pretending we are returning a
valid 32-bit mask.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>