[graphite] Add isOpaque to PrecompileShader

This follows a similar approach to the isAlphaUnchanged function added
to PrecompileColorFilter. Most shaders could be directly implemented
without impacting their number of combinations.

The exceptions to this are solid colors and gradients, which now have 2x
the number of intrinsic combinations to represent where the color (or
all the colors for gradients) are opaque. Similar to
PrecompileColorFilters::Matrix, this increases the combinations
evaluated but does not necessarily trigger a change in the number of
final paint keys (definitely not right now, but possibly impacts the
final blend decision in follow-up CLs).

Bug: b/478239991
Change-Id: I5871fcc274a143e611ce61100c693c5db9b3a90a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1225737
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>