Vectorize scalars in SPIR-V using ConstructorSplat.

This avoids redundant code, and has a small side benefit of
deduplicating constant vectors which appear more than once in the code,
since `writeConstructorSplat` already supports this.

Change-Id: I2972ee922ac92adeb40bc765da3b490a59b957b3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/408360
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
12 files changed