This extends https://crrev.com/c/2917612 also for SIMD, which
(sometimes) uses the special {GetMemOpWithImmOffsetZero} method.
As part of this CL, that method is renamed to {GetEffectiveAddress}
which IMO is a better name. Also, it just returns a register to make the
semantic of that function obvious in the signature.

Drive-by: When sign extending to 32 bit, only write to the W portion of
          the register. This is a bit cleaner, and I first thought that
          this would be the bug.

R=jkummerow@chromium.org
CC=​thibaudm@chromium.org

Bug: chromium:1231950, v8:12018
Change-Id: Ifaefe1f18e3a00534a30c99e3c37ed09d9508f6e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3049073Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75898}