[X86] Add support for reusing ZF etc. from locked XADD instructions (PR20841)

XADD has the same EFLAGS behaviour as ADD

Reapplies rG2149aa73f640 (after it was reverted at rG535df472b042) - AFAICT rG029e41ec9800 should ensure we correctly tag the LXADD* ops as load/stores - I haven't been able to repro the sanitizer buildbot fails locally so this is a speculative commit.