Make dead return statement detection more robust against changes in the CFG.

This change is a precondition to the proposed change to handle temporary
dtors correctly.

The idea is to explicitly search for the next return that doesn't have other
paths into it (that is, if the current block is dead, the block containing the
return must be dead, too). Thus, introducing non-control-flow block
transitions will not break the logic.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209531 91177308-0d34-0410-b5e6-96231b3b80d8