[turbofan] Extend BitcastTaggedToWord(BitcastWordToTaggedSigned(...)) elimination to BitcastTaggedSignedToWord

R=tebbi@chromium.org

Bug: chromium:973047
Change-Id: I33de86258e04140c6da8099a0d9e4aee3f74abe9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1655298
Commit-Queue: Georg Schmid <gsps@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62115}