nir/algebraic: Add more extract_[iu](8|16) optimizations

nir/algebraic: Add more extract_[iu](8|16) optimizations

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>