[GVN] Encode GEPs in offset representation

When using opaque pointers, convert GEPs into offset representation
of the form P + V1 * Scale1 + V2 * Scale2 + ... + ConstantOffset.
This allows us to recognize equivalent address calculations even if
the GEPs don't use the same source element type.

This fixes an opaque pointer codegen regression seen in rustc.

Differential Revision: https://reviews.llvm.org/D124527