nir/algebraic: Be more careful converting ushr to extract_u8/16

If it's not the right bit-size, it may not actually be the correct
extraction.  For now, we'll only worry about 32-bit versions.

Fixes: 905ff861 "nir: Recognize open-coded extract_u16"
Fixes: 76289fbf "nir: Recognize open-coded extract_u8"
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 116b47fe)