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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@13481 91177308-0d34-0410-b5e6-96231b3b80d8