Revert "Revert "Blend functions as SkSL built ins.""

Changes since the last time:

1) Adds workaround for Adreno 6xx devices.

2) Restores ProcessorOptimizationTest threshold to its original values.

This reverts commit 682ba43ba302e4fbe8fe9a9dc45589383e72aed2.

Change-Id: Ic6e65311fc845926fc35c223d6bb13ee2356e16c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/261086
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
12 files changed