[NVPTX] Enable StructuredCFG for NVPTX

Summary:
Make NVPTX require structured CFG. Added a temporary flag to
"roll back" the behavior for easy deployment.

Combined with D45008, this fixes several internal Nvidia GPU test
failures that we suspect to be ptxas miscompiles (PR27738).

Reviewers: jlebar

Subscribers: jholewinski, sanjoy, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D45070

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328885 91177308-0d34-0410-b5e6-96231b3b80d8