clarify can-we-call-JIT logic

I think this makes the relatioship between mask and entry clearer?
Can't have JIT code handle >0 elements unless that JIT code itself
exists.

Change-Id: I238d54a5084c7f90bd32c83db5423840cf415b17
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/222856
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
1 file changed