Add new instruction vpaddl for signed and unsigned integers, adding
assembler, disassembler, simulator support, and also tests.

The signed and unsigned opcodes for vpaddl are separate enums, because
the helper EncodeNeonUnaryOp only takes a NeonSize (not NeonDataType). I
considered changing the signature, but none of the other instructions
use a NeonDataType, so it seems unnecessary.

Bug: v8:11086
Change-Id: I5e6694ae407779c1fd3604c5a40ca0a1b6ce061b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2578233Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71671}