Prevent Constant Folding From Optimizing inrange GEP

This patch does the following things:

1. update SymbolicallyEvaluateGEP so that it bails out if it cannot preserve inrange arribute;
2. update llvm/test/Analysis/ConstantFolding/gep.ll to remove UB in it;
3. remove inaccurate comment above ConstantFoldInstOperandsImpl in llvm/lib/Analysis/ConstantFolding.cpp;
4. add a new regression test that makes sure that no optimizations change an inrange GEP in an unexpected way.

Patch by Zhaomo Yang!

Differential Revision: https://reviews.llvm.org/D51698

llvm-svn: 341888