* Perform the lookups into the builtins constant table and external reference
  table in the generic version of Mov that accepts an Operand source. This
  makes sure we do not miss looking up constants and external references when
  the generic Mov is called.
* Remove Mov(ExternalReference) as ExternalReference can be implicitly converted
  to an Operand.
* Remove two Move functions that are unused in architecture-independent code.
  Replace their uses in arm64-specific code with the generic Mov, which does all
  the necessary work now.

Change-Id: Ibbcee6ba22f661984cd830717e14c9b35a1fba0a
Reviewed-on: https://chromium-review.googlesource.com/1172351
Commit-Queue: Georgia Kouveli <georgia.kouveli@arm.com>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55121}