[AMDGPU] Avoid SCC clobbering before S_CSELECT_B32

[AMDGPU] Avoid SCC clobbering before S_CSELECT_B32

Frame lowering inserts scalar addition to compute the offset to the
stack objects.  This instructions inserted in arbitrary place and may clobber
SCC between its definition and S_CSELECT_B32 instruction.  This change
workarounds this particular code pattern. It queries the scavenger for SGPR and
if available saves SCC to it and restore its value after frame lowering code
insertion.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D136169