Use an offset from TOS for idempotent rmw locked op lowering

This was the portion split off D58632 so that it could follow the redzone API cleanup. Note that I changed the offset preferred from -8 to -64. The difference should be very minor, but I thought it might help address one concern which had been previously raised.

Differential Revision: https://reviews.llvm.org/D61862

llvm-svn: 360719