Remove zero_slot ops from SkRP.

These ops were useful when copy_constant required a separate alloc.
Now that copy_constant can use a packed context, there really isn't
a need for them in the common case. (On a 32-bit machine, they would
still be faster, but these are legacy platforms and I'm not worried
about optimizing for them.)

Change-Id: Ic5335a42f64f79e484b1cede1fcbe768acd27a8f
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/669176
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
193 files changed