Implement halving add idiom (with checker tests).

Rationale:
First of several idioms that map to very efficient SIMD instructions.
Note that the is-zero-ext and is-sign-ext are general-purpose utilities
that will be widely used in the vectorizer to detect low precision
idioms, so expect that code to be shared with many CLs to come.

Test: test-art-host, test-art-target
Change-Id: If7dc2926c72a2e4b5cea15c44ef68cf5503e9be9