[flang][CodeGen] Transform `fir.{store|load}` to `llvm.{store|load}`

This patch extends the `FIRToLLVMLowering` pass in Flang by adding a
hook to transform `fir.store`/`fir.load` to `llvm.store`/`fir.load`,
respectively.

This is part of the upstreaming effort from the `fir-dev` branch in [1].

[1] https://github.com/flang-compiler/f18-llvm-project

Differential Revision: https://reviews.llvm.org/D113090



Patch originally written by:
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>
Co-authored-by: Jean Perier <jperier@nvidia.com>