[ConstantFold] Refactor load folding

This refactors load folding to happen in two cleanly separated
steps: ConstantFoldLoadFromConstPtr() takes a pointer to load from
and decomposes it into a constant initializer base and an offset.
Then ConstantFoldLoadFromConst() loads from that initializer at
the given offset. This makes the core logic independent of having
actual GEP expressions (and those GEP expressions having certain
structure) and will allow exposing ConstantFoldLoadFromConst() as
an independent API in the future.

This is mostly only a refactoring, but it does make the folding
logic slightly more powerful.

Differential Revision: https://reviews.llvm.org/D111023