Use an offset from TOS for idempotent rmw locked op lowering

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360719 91177308-0d34-0410-b5e6-96231b3b80d8