[AMDGPU] Fix computing live registers in prolog

ScratchExecCopy needs to be marked as live, we cannot use that register
while EXEC is stored in there.

Marking SGPRForFPSaveRestoreCopy and SGPRForBPSaveRestoreCopy as
available is unnecessary, they should not be live at that point anway.

Differential Revision: https://reviews.llvm.org/D100098