[IR] Add SaturatingInst and BinaryOpIntrinsic classes

Based on the suggestion in D62447, this adds a SaturatingInst class
that represents the saturating add/sub family of intrinsics. It
exposes the same interface as WithOverflowInst, for this reason I
have also added a common base class BinaryOpIntrinsic that holds the
actual implementation code and will be useful in some places handling
both overflowing and saturating math.

Differential Revision: https://reviews.llvm.org/D62466

llvm-svn: 361857