[SimplifyCFG] delay switch condition forwarding to -latesimplifycfg

As discussed in D39011:
https://reviews.llvm.org/D39011
...replacing constants with a variable is inverting the transform done
by other IR passes, so we definitely don't want to do this early. 
In fact, it's questionable whether this transform belongs in SimplifyCFG 
at all. I'll look at moving this to codegen as a follow-up step.

llvm-svn: 316298