[SystemZFrameLowering] Don't overrwrite R1D (backchain) when probing.

The loop-based probing done for stack clash protection altered R1D which
corrupted the backchain value to be stored after the probing was done.

By using R0D instead for the loop exit value, R1D is not modified.

Review: Ulrich Weigand.

Differential Revision: https://reviews.llvm.org/D92803