simplify-locals: Unify use count visitors
simplify-locals: Unify use count visitors

The simplify locals implementation uses two different visitors to update
the locals use counts. The DeclMarker calculates the initial use counts.
The StatementDeclMarker updates the use counts as statements are being
removed from the block.

Replace them with a single visitor that can operate in either mode,
ensuring consistency of behaviour.

Additionally use exhaustive match to clarify what is being optimized.

No functional changes intended.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust