Refactors bytecode register access to avoid having to deal with register indexes
directly.

 - Changes Load/StoreRegister to Load/StoreRegisterAtOperandIndex
 - Adds RegisterList abstraction for dealin with lists of registers
 - Adds helpers for Loading / Storing register pairs / triples.

Change-Id: I34427e4bd7314dce0230572212580d6a93ccc2d4
Reviewed-on: https://chromium-review.googlesource.com/887062Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50899}