Fix fixes a minor DCHECK error, which fired because usually only
LiftoffRegisters of the same type should be compared against each
other. In the RegisterReuseMap we store both register pairs and single
registers, hence check for pair vs non-pair comparisons explicitly.

R=ahaas@chromium.org

Bug: v8:6600, chromium:922670
Change-Id: I18beb61b6b1906cc42bcf6e7dfdd5eb803e874d7
Reviewed-on: https://chromium-review.googlesource.com/c/1421921Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58956}