Fix PR7328: when turning a tail recursion into a loop, need to preserve

Fix PR7328: when turning a tail recursion into a loop, need to preserve
the returned value after the tail call if it differs from other return
values.  The optimal thing to do would be to introduce a phi node for
the return value, but for the moment just fix the miscompile.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106947 91177308-0d34-0410-b5e6-96231b3b80d8