nir/algebraic: Remove spurious conversions from inside logic ops

Not only does this eliminate a bunch of unnecessary type converting
MOVs, but it can also enable some SWAR.  The
dEQP-VK.spirv_assembly.type.vec3.i8.bitwise_xor_frag test does
something about like:

    c = a.x ^ b.x;
    d = a.y ^ b.y;
    e = a.z ^ b.z;

After this change, it looks more like:

    uint t = i8vec3AsUint(a) ^ i8vec3AsUint(b);
    c = extract_u8(t, 0);
    d = extract_u8(t, 1);
    e = extract_u8(t, 2);

On Ice Lake, this results in:

SIMD8 shader: 41 instructions. 1 loops. 3804 cycles. 0:0 spills:fills, 5 sends
SIMD8 shader: 31 instructions. 1 loops. 2844 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>
1 file changed