Remove an instance where the 'unwind' instruction was created.

Remove an instance where the 'unwind' instruction was created.

The 'unwind' instruction was acting essentially as a placeholder, because it
would be replaced at the end of this function by a branch to the "unwind
handler". The 'unwind' instruction is going away, so use 'unreachable' instead,
which serves the same purpose as a placeholder.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137098 91177308-0d34-0410-b5e6-96231b3b80d8