[Attributor] Track a GEP Instruction in align deduction
Summary:
This patch enables us to track GEP instruction in align deduction.
If a pointer `B` is defined as `A+Offset` and known to have alignment `C`, there exists some integer Q such that
```
A + Offset = C * Q = B
```
So we can say that the maximum power of two which is a divisor of gcd(Offset, C) is an alignment.
Reviewers: jdoerfert, sstefan1
Reviewed By: jdoerfert
Subscribers: lebedev.ri, hiraditya, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D70392