JIT: Refactor physical promotion in preparation of generalized assignment decompositi...

JIT: Refactor physical promotion in preparation of generalized assignment decomposition (#85728)

This refactors the physical promotion to make the generalized assignment
decomposition PR more manageable to review.

* Pull Replacement and ReplaceVisitor into the header
* Introduce promotiondecomposition.cpp that deals with handling
  assignments, and move relevant code from promotion.cpp to this file
* Rename StatementList -> DecompositionStatementList and unnest it
* Rename Replacement::Name -> Replacement::Description which is more fitting

Also adds some more JITDUMPs when picking promotions, but otherwise
should not affect behavior.