intel/fs: Fix D to W conversion in opt_combine_constants
Found by GCC warning:
src/intel/compiler/brw_fs_combine_constants.cpp: In function ‘bool needs_negate(const fs_reg*, const imm*)’: src/intel/compiler/brw_fs_combine_constants.cpp:306:34: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits] return ((reg->d & 0xffffu) < 0) != (imm->w < 0); ~~~~~~~~~~~~~~~~~~~^~~
The result of the bit-and is a 32-bit value with the top bits all zero. This will never be < 0. Instead of masking off the bits, just cast to int16_t and let the compiler handle the actual conversion.
Passes Intel CI. Since the incorrect code also passed, this just means we have inadequate test coverage. I'm open for suggestions to improve that. Perhaps a targeted unit test in Mesa?