AMDGPU: Avoid overwriting saved PC

Summary:
An outstanding load with same destination sgpr as call could cause PC to be
updated with junk value on return.

Reviewers: arsenm, rampitec

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69474