Allow basicaa to walk through geps with identical indices in

Allow basicaa to walk through geps with identical indices in
parallel, allowing it to decide that P/Q must alias if A/B
must alias in things like:
 P = gep A, 0, i, 1
 Q = gep B, 0, i, 1

This allows GVN to delete 62 more instructions out of 403.gcc.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60820 91177308-0d34-0410-b5e6-96231b3b80d8