Add RP codegen support for the 'dot' intrinsic.

This boils down very succinctly into the following pattern:
- 2 slots: mul_2, add
- 3 slots: mul_3, add, add
- 4 slots: mul_4, add_2, add

Change-Id: If5b0ee6e2d7f51da803fe5fd6882d4c9b1c13206
Bug: skia:13676
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/621978
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
2 files changed