[ConstantFolding] Add support for saturating add/sub

Support saturating add/sub in constant folding, based on the APInt methods introduced in D54332.

Patch by: @nikic (Nikita Popov)

Differential Revision: https://reviews.llvm.org/D54531


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347328 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed