[fir] Split FIROptimizer lib into several smaller libraries

Partition libFIROptimizer into smaller libraries that reflect the
structure. Adapt potential problems.

This patch is part of the upstreaming effort from fir-dev branch. It's a
building stone to upstreaming transformations.

Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>

Reviewed By: schweitz

Differential Revision: https://reviews.llvm.org/D111055