Seperates the memory lowering operations into a seperate MemoryLowering
class which is used by the MemoryOptimizer. This will enable TurboProp
to reduce memory operations without having to do a full memory
optimization pass.

BUG=v8:9684

Change-Id: I1b333f1360fd342612672842bf879f44ab1ee60c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1815243Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63966}