executeScopConditionally: Introduce special exiting block

When introducing separate control flow for the original and optimized code we
introduce now a special 'ExitingBlock':

      \   /
    EnteringBB
        |
    SplitBlock---------\
   _____|_____         |
  /  EntryBB  \    StartBlock
  |  (region) |        |
  \_ExitingBB_/   ExitingBlock
        |              |
    MergeBlock---------/
        |
      ExitBB
      /    \

This 'ExitingBlock' contains code such as the final_reloads for scalars, which
previously were just added to whichever statement/loop_exit/branch-merge block
had been generated last. Having an explicit basic block makes it easier to
find these constructs when looking at the CFG.

llvm-svn: 255107