- Adds CodeGenerator::NeonInputOperand method for arm.
- Specifies addressing mode when selecting Neon load/store.

Bug: 
Change-Id: Iac0bb566c32882e6b9605d2978588af05493a5c7
Reviewed-on: https://chromium-review.googlesource.com/716714
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48562}