Generate substantially better code when there are a limited number of exits

from the extracted region.  If the return has 0 or 1 exit blocks, the new
function returns void.  If it has 2 exits, it returns bool, otherwise it
returns a ushort as before.

This allows us to use a conditional branch instruction when there are two
exit blocks, as often happens during block extraction.

llvm-svn: 13481