DX: Add support for invoke-polymorphic.

This change identifies virtual calls to signature polymorphic methods
and converts the instruction from invoke-virtual to invoke-polymorphic.

Enabled if --min-sdk-version >= 26.

Bug: 30550796,33191712
Test: dx/tests/run-test 132-invoke-polymorphic
Change-Id: I6771d43982f1b72db030b0c77285bb8e04f918b9