Instcombine constant folding can normalize gep with negative index to index with...

Instcombine constant folding can normalize gep with negative index to index with large offset. When instcombine objsize checking transformation sees these geps where the offset seemingly point out of bound, it should just return "i don't know" rather than asserting.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96825 91177308-0d34-0410-b5e6-96231b3b80d8