Merging r278575 (with changes to the test):

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r278575 | haicheng | 2016-08-12 16:13:38 -0700 (Fri, 12 Aug 2016) | 6 lines

Reapply [BranchFolding] Restrict tail merging loop blocks after MBP

Fixed a bug in the test case.

To fix PR28104, this patch restricts tail merging to blocks that belong to the
same loop after MBP.
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I had to adjust the test as it wasn't passing on the branch, presumably
due to different machine block placement.

llvm-svn: 278827