This fixes the Crucible func.shader.shift.int8_t test on Gen8 and Gen9.
See mesa/crucible!76

.

With the previous optimizations in place, this change seems to improve
the quality of the generated code.  Comparing a couple Vulkan CTS tests
on Skylake had the following results.

dEQP-VK.spirv_assembly.type.vec3.i8.bitwise_xor_frag:
SIMD8 shader: 36 instructions. 1 loops. 3822 cycles. 0:0 spills:fills, 5 sends
SIMD8 shader: 27 instructions. 1 loops. 2742 cycles. 0:0 spills:fills, 5 sends

dEQP-VK.spirv_assembly.type.vec3.i8.max_frag:
SIMD8 shader: 39 instructions. 1 loops. 3922 cycles. 0:0 spills:fills, 5 sends
SIMD8 shader: 37 instructions. 1 loops. 3682 cycles. 0:0 spills:fills, 5 sends

Reviewed-by: Francisco Jerez <currojerez@riseup.net>