AMDGPU: Propagate undef flag during pre-RA exec mask optimizations

Summary: Issue: https://github.com/GPUOpen-Drivers/llpc/issues/204

Reviewers: arsenm, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68184

llvm-svn: 374041