[ARM] Basic gather scatter cost model

This is a very basic MVE gather/scatter cost model, based roughly on the
code that we will currently produce. It does not handle truncating
scatters or extending gathers correctly yet, as it is difficult to tell
that they are going to be correctly extended/truncated from the limited
information in the cost function.

This can be improved as we extend support for these in the future.

Based on code originally written by David Sherwood.

Differential Revision: https://reviews.llvm.org/D73021