generalize binop reassociation; NFC

Move the fold introduced in r236031:
http://reviews.llvm.org/rL236031

to its own helper function, so we can use it for other binops.

This is a preliminary step before partially solving:
https://llvm.org/bugs/show_bug.cgi?id=21768
https://llvm.org/bugs/show_bug.cgi?id=23116

llvm-svn: 236171