Simplify the loop-unswitch pass, by not even trying to unswitch loops with

Simplify the loop-unswitch pass, by not even trying to unswitch loops with
uses of loop values outside the loop.  We need loop-closed SSA form to do
this right, or to use SSA rewriting if we really care.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26089 91177308-0d34-0410-b5e6-96231b3b80d8