[APInt] Add saturating multiply ops

Summary:
There are `*_ov()` functions already, so at least for consistency it may be good to also have saturating variants.
These may or may not be needed for `ConstantRange`'s `mulWithNoWrap()`

Reviewers: spatel, nikic

Reviewed By: nikic

Subscribers: hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69397