JIT: refactor escape analysis to be more bv-centric (#115213)

Make the "bv index" the primary concept for computations; translate
from local var nums to their indices when needed.

Allocate all the connection BVs at once, since the connection matrix
will now have a dense set of entries.

This is prep work for tracking fields of stack allocatable objects,
where we will introduce new classes of things tracked by the
connection graph.

Author: AndyAyersMS