Use `writeComposite` to vectorize scalars.

`writeComposite` can write and deduplicate constants, so it's preferable
to manually emitting an OpCompositeConstruct opcode.

Change-Id: Ie5c23af76822da762eadac8ff0ab0c6cc0febd31
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/528637
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
6 files changed