intel/fs: Emit better code for u2u of extract

intel/fs: Emit better code for u2u of extract

Emitting the instructions one by one results in two MOV instructions
that won't be propagated.  By handling both instructions at once, a
single MOV is emitted.  For example, on Ice Lake this helps
dEQP-VK.spirv_assembly.type.vec3.i8.bitwise_xor_frag:

SIMD8 shader: 49 instructions. 1 loops. 4044 cycles. 0:0 spills:fills, 5 sends
SIMD8 shader: 41 instructions. 1 loops. 3804 cycles. 0:0 spills:fills, 5 sends

Without "intel/fs: Allow copy propagation between MOVs of mixed sizes,"
the improvement is still 8 instructions, but there are more instructions
to begin with:

SIMD8 shader: 52 instructions. 1 loops. 4164 cycles. 0:0 spills:fills, 5 sends
SIMD8 shader: 44 instructions. 1 loops. 3944 cycles. 0:0 spills:fills, 5 sends

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