Codegen: Don't tail-duplicate blocks with un-analyzable fallthrough.
If AnalyzeBranch can't analyze a block and it is possible to
fallthrough, then duplicating the block doesn't make sense, as only one
block can be the layout predecessor for the un-analyzable fallthrough.
Submitted wit a test case, but NOTE: the test case doesn't currently
fail. However, the test case fails with D20505 and would have saved me
some time debugging.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278288
91177308-0d34-0410-b5e6-
96231b3b80d8
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