This removes a redundant SpillRegisters implementation in
liftoff-assembler-ia32.h, and uses the existing platform-independent
method in liftoff-assembler.h consistently.

R=ahaas@chromium.org

Bug: v8:11384
Change-Id: I20f0417c248e06557dc1fe8bab834a99257940de
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2695399Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72750}