EncodeNeonUnaryOp and EncodeNeonSizedOp are essentially the same, they
are both helpers to encode instructions in the encoding group "Advanced
SIMD two registers misc". Consolidate them into a single helper.

Bug: v8:11074
Change-Id: I8b0c21bfbce51de72b41cbd3d34cf4fed9ce8819
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2567535
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71587}