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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278575 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed