Simplify code a bit, and fix bug CodeExtractor/2004-03-14-NoSwitchSupport.ll

This also implements a two minor improvements:
  * Don't insert live-out stores IN the region, insert them on the code path
    that exits the region
  * If the region is exited to the same block from multiple paths, share the
    switch statement entry, live-out store code, and the basic block.

llvm-svn: 12401