[AMDGPU] Enable merging m0 initializations.

Summary:
Enable hoisting and merging m0 defs that are initialized with the same
immediate value. Fixes bug where removed instructions are not considered
to interfere with other inits, and make sure to not hoist inits before block
prologues.

Reviewers: rampitec, arsenm

Reviewed By: rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64766

llvm-svn: 366135