Merging r328885:

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r328885 | timshen | 2018-03-30 10:51:03 -0700 (Fri, 30 Mar 2018) | 14 lines

[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
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llvm-svn: 333475