[InstCombine] add/use overflowing math helper functions; NFC

The mul case can already be refactored to use this similar to
rL342278.
The sub case is proposed in D52075.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@342289 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed