Basic SIMD reduction support.

Rationale:
Enables vectorization of x += .... for very basic (simple, same-type)
constructs. Paves the way for more complex (narrower and/or mixed-type)
constructs, which will be handled by the next CL.

Test: test-art-host test-art-target

Bug: 64091002

Change-Id: I7880c135aee3ed0a39da9ae5b468cbf80e613766