StructurizeCFG: Relax uniformity checks.

This change relaxes the checks for hasOnlyUniformBranches such that our
region is uniform if:

1. All conditional branches that are direct children are uniform.
2. And either:
  a. All sub-regions are uniform.
  b. There is one or less conditional branches among the direct
     children.

Differential Revision: https://reviews.llvm.org/D62198

llvm-svn: 361610