In order to support i64 values on 32 bit platforms, we extend the
{LiftoffRegister} class to support storing pairs of GP registers on
those platforms. On 64 bit platforms, this refactoring should cause no
slowdown, as the compiler can statically determine that
{LiftoffRegister::is_pair()} is always false.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ie9966d599271b8779959b1809ab4e129b68b80d7
Reviewed-on: https://chromium-review.googlesource.com/890261Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50986}