nir/algebraic: Optimize some extract forms resulting from 8-bit lowering

nir/algebraic: Optimize some extract forms resulting from 8-bit lowering

This eliminates some spurious, size-converting moves.  For example, on
Ice Lake this helps dEQP-VK.spirv_assembly.type.vec3.i8.bitwise_xor_frag:

SIMD8 shader: 56 instructions. 1 loops. 4444 cycles. 0:0 spills:fills, 5 sends
SIMD8 shader: 52 instructions. 1 loops. 4164 cycles. 0:0 spills:fills, 5 sends

v2: Condition two of the patterns on !options->lower_extract_byte.
Suggested by Lionel.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9025>