comment and TODO bilerp code
comment and TODO bilerp code

I'm starting my way towards AVX2 code here, first starting with the
relatively simple SSE2 code to make sure I understand what's going on.

Renames, updated comments, and one performance-related TODO.

Change-Id: I366ee67cf89c8c8d2ccc610d6d753531a131e7b9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/243815
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Auto-Submit: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: Mike Reed <f5cabf8735907151a446812c9875d6c0c712d847@google.com>

HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia