Represent vector construction with ConstructorVector.

This constructor aggregates scalars and smaller vectors together into
one vector. It is not responsible for splats or typecasts; those are
handled in separate classes which were added previously.

Change-Id: I9194bec50d58d94c331e0bb883686e26b86af347
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/392816
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
19 files changed