Merging r330345:

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r330345 | kparzysz | 2018-04-19 10:26:46 -0700 (Thu, 19 Apr 2018) | 8 lines

[if-converter] Handle BBs that terminate in ret during diamond conversion

This fixes https://llvm.org/PR36825.

Original patch by Valentin Churavy (D45218).

Differential Revision: https://reviews.llvm.org/D45731

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llvm-svn: 333561