[InstCombine] refactor isCheapToScalarize(); NFC

As the FIXME indicates, this has the potential to go
overboard. So I'm not sure if it's even worth keeping 
this vs. iteratively doing simple matches, but we might 
as well clean it up.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349523 91177308-0d34-0410-b5e6-96231b3b80d8