Introduce ConstantFoldBinaryOpOperands function and migrate some callers of ConstantFoldInstOperands to use it. NFC.
Although this is a slight cleanup on its own, the main motivation is to
refactor the constant folding API to ease migration to opaque pointers.
This will be follow-up work.
Subscribers: dblaikie, llvm-commits
Differential Revision: http://reviews.llvm.org/D16378