[InstSimplify] Simplify saturating add/sub + icmp

If a saturating add/sub has one constant operand, then we can
determine the possible range of outputs it can produce, and simplify
an icmp comparison based on that.

The implementation is based on a similar existing mechanism for
simplifying binary operator + icmps.

Differential Revision: https://reviews.llvm.org/D55735

llvm-svn: 349369