SimplifyCFG: Teach switch generation some patterns that instcombine forms.

This allows us to create switches even if instcombine has munged two of the
incombing compares into one and some bit twiddling. This was motivated by enum
compares that are common in clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185632 91177308-0d34-0410-b5e6-96231b3b80d8