RegisterLists should only be allocated via the register allocator. To ensure
this, make the RegisterList constructor private and only expose it to tests
and the BytecodeRegisterAllocator.

Change-Id: I09ebfc5c0f1baecfb1333fd672b96d462fd26fcf
Reviewed-on: https://chromium-review.googlesource.com/822196
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50073}