[InstCombine] Merge duplicate functionality between InstCombine and ValueTracking

[InstCombine] Merge duplicate functionality between InstCombine and ValueTracking

Summary:
Merge overflow computation for signed add,
appearing both in InstCombine and ValueTracking.

As part of the merge,
cleanup the interface for overflow checks in InstCombine.

Patch by Yoav Ben-Shalom.

Reviewers: craig.topper, majnemer

Reviewed By: craig.topper

Subscribers: takuto.ikuta, llvm-commits

Differential Revision: https://reviews.llvm.org/D32946

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303029 91177308-0d34-0410-b5e6-96231b3b80d8