This adds the "(a << N) >> M" family of mask or sign-extensions. Not a
huge win right now but this pattern will soon be generated by NIR format
lowering code.
Shader-db results on Kaby Lake:
total instructions in shared programs: 15166918 -> 15166916 (<.01%)
instructions in affected programs: 36 -> 34 (-5.56%)
helped: 2
HURT: 0
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Jason Ekstrand
4dd5263663