Small refactoring to use an existing ElementSizeInPointers (which has
the same implementation). LinkageLocation::GetSize is now not called
anywhere else, so remove it. The instruction selector can then call
LinkageLocation::GetSizeInPointers instead of calculating it.

Bug: v8:11879
Change-Id: I6f717db1c4047e7847f00cf2bf96c6ec8ea52bda
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2997924Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75520}