[AMDGPU] Fix missed SI_RETURN_TO_EPILOG in pre-emit peephole

SIPreEmitPeephole does not process all terminators, which means
it can fail to handle SI_RETURN_TO_EPILOG if immediately preceeded
by a branch to the early exit block.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D85872