AMDGPU: Move m0 initializations earlier

Summary:
After hoisting and merging m0 initializations schedule them as early as
possible in the MBB. This helps the scheduler avoid hazards in some
cases.

Reviewers: rampitec, arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67450

llvm-svn: 371671