AMDGPU: M0 operands to spill/restore opcodes are dead

Summary:
With scalar stores, M0 is clobbered and therefore marked as implicitly
defined. However, it is also dead.

This fixes an assertion when the Greedy Register Allocator decides to
optimize a spill/restore pair away again (via tryHintsRecoloring).

Reviewers: arsenm

Subscribers: qcolombet, kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D33319

llvm-svn: 306375