[LV] Preserve inbounds on created GEPs

Summary:
This is a fix for PR23997.

The loop vectorizer is not preserving the inbounds property of GEPs that it creates.
This is inhibiting some optimizations. This patch preserves the inbounds property in
the case where a load/store is being fed by an inbounds GEP.

Reviewers: mkuper, javed.absar, hsaito

Reviewed By: hsaito

Subscribers: dcaballe, hsaito, llvm-commits

Differential Revision: https://reviews.llvm.org/D46191

llvm-svn: 331269