We introduce an explicit LoweringResult data structure. Until this change,
the lowering result could be recovered from the node. However, lowering
monomorphic loads requires wiring different value and effect, so we need
a structure that can express such lowering result.

Bug: v8:6357
Change-Id: I92655800890b744d9203a778a1936a8dcd465ed3
Reviewed-on: https://chromium-review.googlesource.com/637304
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47992}