[PowerPC] Spilling to registers does not require frame index scavenging

If spills are to registers instead of to the stack then a copy will be used
and frame index scavenging is not required.

This patch adds debug info to frame index scavenging and makes sure that
spilling to registers does not cause frame index scavenging.

Reviewed By: nemanjai, #powerpc

Differential Revision: https://reviews.llvm.org/D101360