Claim NoAlias if two GEPs index different fields of the same struct

Patch by Taewook Oh

Summary: Patch for Bug 27478. Make BasicAliasAnalysis claims NoAlias if two GEPs index different fields of the same structure.

Reviewers: hfinkel, dberlin

Subscribers: dberlin, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D20665

llvm-svn: 271415