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.

This is a revert^2 of I7880c135aee3ed0a39da9ae5b468cbf80e613766
and thus a revert  of I1c1c87b6323e01442e8fbd94869ddc9e760ea1fc

PS1-2 shows what needed to change, with regression tests

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

Bug: 64091002, 65212948
Change-Id: I2454778dd0ef1da915c178c7274e1cf33e271d0f