This allows to emit the best opcode for the register move. The type is
available at all call sites anyway.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I8516deff4d8a5480cea9df37cfc003fb9c668e8c
Reviewed-on: https://chromium-review.googlesource.com/910910Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51210}