Don't optimize the landing pad exit block.

Don't optimize the landing pad exit block.

One way to exit the loop is through an unwind edge. However, that may involve
splitting the critical edge of the landing pad, which is non-trivial. Prevent
the transformation from rewriting the landing pad exit loop block.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137871 91177308-0d34-0410-b5e6-96231b3b80d8