[InstSimplify] add tests for saturating add/sub; NFC

These are baseline tests for D54532.
Patch based on the original tests by:
@nikic (Nikita Popov)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347060 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed