fix rdar://8785296 - -fcatch-undefined-behavior generates inefficient code

fix rdar://8785296 - -fcatch-undefined-behavior generates inefficient code

The basic issue is that isel (very reasonably!) expects conditional branches
to be folded, so CGP leaving around a bunch dead computation feeding
conditional branches isn't such a good idea.  Just fold branches on constants
into unconditional branches.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123526 91177308-0d34-0410-b5e6-96231b3b80d8