Add Float32Select and Float64Select as OptionalOperators and insert
these, if supported, when handling a Select expression in the wasm
graph builder. FlagsContinuation have been modified to support the
select operation and code generation support has been added for arm64.

This improves the 'Bullet' physics benchmark by ~2-3%.

Change-Id: I928c3085c9136ad8baeeb34c71c47c1c8338844c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2763871
Commit-Queue: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73657}