Fix step & smoothstep on Vulkan

Both of these have variants with mixed scalar/vector parameters. Ensure
that all parameters are vectorized, or we fail SPIR-V validation.

Bug: skia:10913
Change-Id: I1a5be7fc02695e4c7047b5b9c3c08d12b2071e21
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/334896
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
2 files changed