[NFC][CodeGen] Use the ExitBlock explicitly.

Before we would 'guess' the correct location for the MergeBlock
that got introduced when executing a Scop conditionally. This
implicitly depends on the situation that at this point during
CodeGen there will be nothing between polly.start and polly.exiting.

With this commit we explicitly state that we want the block that
directly follows polly.exiting.

llvm-svn: 306398