Support for narrow operands in "dangerous" operations.

Rationale:
Under strict conditions, even operations that are sensitive
to higher order bits can vectorize by inspecting the operands
carefully. This enables more vectorization, as demonstrated
by the removal of quite a few TODOs.

Test: test-art-target, test-art-host
Change-Id: I2b0fda6a182da9aed9ce1708a53eaf0b7e1c9146