[BasicAA] Make sure context instruction is symmetric

D71264 started using a context instruction in a computeKnownBits()
call. However, if aliasing between two GEPs is checked, then the
choice of context instruction will be different for alias(GEP1, GEP2)
and alias(GEP2, GEP1), which is not supposed to happen.

Resolve this by remembering which GEP a certain VarIndex belongs to,
and use that as the context instruction. This makes the choice of
context instruction predictable and symmetric.

It should be noted that this choice of context instruction is
non-optimal (just like the previous choice): The AA query result is
only valid at points that are reachable from *both* instructions.
Using either one of them is conservatively correct, but a larger
context may also be valid to use.

Differential Revision: https://reviews.llvm.org/D93183