MIPS: Basic SIMD reduction support.

Enables vectorization of x += .... for very basic (simple, same-type)
constructs for MIPS.

Note: Testing is done with checker parts of tests 661 and 665,
      locally changed to cover MIPS32 cases. These changes can't
      be included in this patch since MSA is not a default option.

Test: test-art-host test-art-target
Change-Id: Ia3b3646afecb76c2f00996a30923ca70302be57e