StructurizeCFG: Simplify inserted PHI nodes

Summary:
This improves subsequent divergence analysis in some cases.

Change-Id: I5e95e7ec7fd3fa80d414d1a53a02fea23e3d67d3

Reviewers: arsenm, rampitec

Subscribers: jvesely, wdng, llvm-commits

Differential Revision: https://reviews.llvm.org/D53316

llvm-svn: 344697