- Adds opcode for 32/16/8 bit dup instruction.
- Matches shuffles that are equivalent to dup's.

Bug: v8:6020
Change-Id: I8848d974adf30127d1dc31c09a9517f8f9573ce9
Reviewed-on: https://chromium-review.googlesource.com/571448
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46803}