In gcov profiling, give all functions an extra unified return block. This is

In gcov profiling, give all functions an extra unified return block. This is
necessary since gcov counts transitions between blocks. It can't see if you've
run every line in a straight-line function, so we add an edge for it to notice.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@129905 91177308-0d34-0410-b5e6-96231b3b80d8