Cleanup code to factor out bit-checks on register allocations to a
seperate RegisterBitVector class.

BUG=v8:9684

Change-Id: I33306a858da252d0be76eecaa9ea47b9b53f088b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2464936Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70446}