[AMDGPU] Fix Livereg computation during epilogue insertion

The LivePhysRegs calculated in order to find a scratch register in the
epilogue code wrongly uses 'LiveIns'. Instead, it should use the
'Liveout' sets.  For the liveness, also considering the operands of
the terminator (return) instruction which is the insertion point for
the scratch-exec-copy instruction.

Patch by Christudasan Devadasan

llvm-svn: 364470