Allocating a temp register in a conditional branch confuses
the LiftoffAssembler's state tracking, so this patch moves
allocation of the register into the unconditional part of the
control flow.

Fixed: chromium:1275711
Change-Id: Ic83ba8c098c5edb33d035c1a93931d54cc1f1caa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3320423
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78275}