Pair operations needed more than 5 registers; this allows one operand
to go into a slot or an immediate, which allows pair operations even if
ebx if not allocatable because it holds the root register.

Change-Id: Ic08b2e0ca93744a434e3d41af97da45b779756aa
Bug: v8:6666
Reviewed-on: https://chromium-review.googlesource.com/c/1271055
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56497}