[X86] Use OR32mi8Locked instead of LOCK_OR32mi8 in emitLockedStackOp.

They encode the same way, but OR32mi8Locked sets hasUnmodeledSideEffects set
which should be stronger than the mayLoad/mayStore on LOCK_OR32mi8. I think
this makes sense since we are using it as a fence.

This also seems to hide the operation from the speculative load hardening pass
so I've reverted r360511.

llvm-svn: 360747