Track single def locals in importer (#21251)

Track single def locals in importer (#21251)

* Defer setting lvSingleDef until lvaMarkLocalVars

Since this information isn't used early on in the jit, defer setting this bit
until later.

* Track single-def locals in the importer

and use this information to enable type updates during late devirtualization.
This gets some cases where the exact type materializes late (perhaps via a
chain of copies).