Machines lacking AND-NOT should transform `(a & x) | (~a & y)` -> `(a & (x ^ y)) ^ y`

[Validark]

Machines lacking an AND-NOT instruction should transform select into select2. Godbolt link

#include <stdint.h>

uint64_t select(uint64_t a, uint64_t x, uint64_t y) {
    return (a & x) | (~a & y);
}

uint64_t select2(uint64_t a, uint64_t x, uint64_t y) {
    return (a & (x ^ y)) ^ y;
}

RISC-V emit:

select:                                 # @select
        and     a1, a1, a0
        not     a0, a0
        and     a0, a0, a2
        or      a0, a0, a1
        ret
select2:                                # @select2
        xor     a1, a1, a2
        and     a0, a0, a1
        xor     a0, a0, a2
        ret

[pinskia]

Note GCC's bugzilla about this was https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63568 .

[RKSimon]

We already do this in X86 as part of foldMaskedMerge - it shouldn't be difficult to make this generic if we can control it with TLI.hasAndNot