Allow type based sharpening for imprecise references.

When a reference is imprecise we may still be able to devirtualize if the
method that is dispatched upon may not be overridden (ie final).
Some other tidying of the devirtualization code.

For boot this increases the frequency of type based devirtualization by a
little more than 0.01%.

Change-Id: I050efbcc78c6b89135a6432bd7c2e946d8efbab4