[InstCombine] simplify binops before trying other folds

This is outwardly NFC from what I can tell, but it should be more efficient 
to simplify first (despite the name, SimplifyAssociativeOrCommutative does
not actually simplify as InstSimplify does - it creates/morphs instructions).

This should make it easier to refactor duplicated code that runs for all binops.

llvm-svn: 335258