[X86] Use h-register for final XOR of __builtin_parity on 64-bit targets.

This adds an isel pattern and special XOR8rr_NOREX instruction
to enable the use of h-registers for __builtin_parity. This avoids
a copy and a shift instruction. The NOREX instruction is in case
register allocation doesn't use the matching l-register for some
reason. If a R8-R15 register gets picked instead, we won't be
able to encode the instruction since an h-register can't be used
with a REX prefix.

Fixes PR46954