ART: Add unchecked conversions of arrays

Add array conversion functions that do not check the type,
and use them in an unchecked version for PointerArray.

Decreases dex2oatd preopting of a big app from 151s to 150s.

Bug: 123888325
Test: m test-art-host
Change-Id: Id58c65ac603a20c2cfb9e3289af4f3bc323554b6