Stop maintaining cost estimates in the RyuJIT backend. (#6849)

Stop maintaining cost estimates in the RyuJIT backend. (#6849)

* Summarize HIR cost estimates on Compiler.

These cost estimates are made on HIR and are not used by the backend.
Stash them before converting the function to LIR and report them at the
end of compilation.

* Stop maintaining costs in the backend.

The backend does not use the tree cost estimates.

* Remove a bit more cost maintenance.

* Address PR feedback.