[NFC] Return both polly.start and polly.exiting from executeScopConditionally.

This commit returns both the start and the exit block that are created
by executeScopConditionally.

In a future commit we will make use of the exit block. Before we would
have to use the implicit property that there won't be any code generated
between polly.start and polly.exiting at the time of use to find the
correct block ('polly.exiting').

All usage location are semantically unchanged.

llvm-svn: 306283