[StructurizeCFG] Enable -structurizecfg-relaxed-uniform-regions by default

D62198 introduced an option to relax the checks for
hasOnlyUniformBranches. This commit turns the option on by default, for
better code generation in some cases in AMDGPU.

Differential Revision: https://reviews.llvm.org/D63198

Change-Id: I9cbff002a1e74d3b7eb96b4192dc8129936d537d
llvm-svn: 368042