SIMD shifts was changed to use register in https://crrev.com/c/1722198
so all this code to deal with shift immediates can be removed.

SimdOp was also removed from the interface, so now Liftoff does not need
to implement it.

Bug: v8:9810
Change-Id: I845e28b4f5712ff188d68beea7121eebddebdd9e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1913506Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64944}