[BasicAA] Generalize base offset modulus handling

The GEP aliasing implementation currently has two pieces of code

that solve two different subsets of the same basic problem: If you

have GEPs with offsets 4*x + 0 and 4*y + 1 (assuming access size 1),

then they do not alias regardless of whether x and y are the same.

One implementation is in aliasSameBasePointerGEPs(), which looks at

this in a limited structural way. It requires both GEP base pointers

to be exactly the same, then (optionally) a number of equal indexes,

then an unknown index, then a non-equal index into a struct. This

set of limitations works, but it's overly restrictive and hides the

core property we're trying to exploit.

The second implementation is part of aliasGEP() itself and tries to

find a common modulus in the scales, so it can then check that the

constant offset doesn't overlap under modular arithmetic. The second

implementation has the right idea of what the general problem is,

but effectively only considers power of two factors in the scales

(while aliasSameBasePointerGEPs also works with non-pow2 struct sizes.)

What this patch does is to adjust the aliasGEP() implementation to

instead find the largest common factor in all the scales (i.e. the GCD)

and use that as the modulus.

Differential Revision: https://reviews.llvm.org/D91027