Convert constant variables to values earlier in intrinsic optimization.
Convert constant variables to values earlier in intrinsic optimization.

It's dangerous to leave constant variable expressions lying around, as
the temptation to access them via getConstantSubexpression is just too
great, but this currently isn't supported. This bit us in `inverse`
(see http://review.skia.org/440260) and it seems like we had a similar
latent bug with `cross` as well.

Fortunately, this change seemed to simplify the code in general.

Change-Id: Idd3e84ca202a1f05f91c1d6d5e94a3bc6da0d8d7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/447016
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Commit-Queue: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

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