SI-8823 Exclude specialized methods from extension method rewrite

If a value class extends a specialized class, it can sprout
specialized members after the specialization info transformer has run.
However, we only install extension methods for class members we know
about at the extmethods phase.

This commit simply disables rewiring calls to these methods in
erasure to an extention method. This follows the approach taken
from super accessors.

Note: value class type parameters themselves currently are not
allowed to be specialized.

Review by @gkossakowski