[ARM] Rejig some of the MVE gather/scatter lowering pass. NFC

This adjusts some of how the gather/scatter lowering pass passes around
data and where certain gathers/scatters are created from. It should not
effect code generation on its own, but allows other patches to more
clearly reason about the code.

A number of extra test cases were also added for smaller gathers/
scatters that can be extended, and some of the test comments were
updated.