[AMDGPU] Add llvm.amdgcn.softwqm intrinsic

Add llvm.amdgcn.softwqm intrinsic which behaves like llvm.amdgcn.wqm
only if there is other WQM computation in the shader.

Reviewers: nhaehnle, tpr

Reviewed By: nhaehnle

Subscribers: arsenm, kzhuravl, jvesely, wdng, yaxunl, dstuttard, t-tye, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64935

llvm-svn: 367097