Small improvements to code generated on arm and arm64.

Bug: v8:6644
Change-Id: Id3d9be87f6a071e949985a1b6b62a43d9195a104
Reviewed-on: https://chromium-review.googlesource.com/615300Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Cr-Commit-Position: refs/heads/master@{#47359}