[AST] Fix a bug in aliasesUnknownInst. Make sure we are comparing the unknown...

[AST] Fix a bug in aliasesUnknownInst. Make sure we are comparing the unknown instructions in the alias set and the instruction interested in.

Summary:
Make sure we are comparing the unknown instructions in the alias set and the instruction interested in.
I believe this is clearly a bug (missed opportunity). I can also add some test cases if desired.

Reviewers: hfinkel, davide, dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34597

llvm-svn: 306241