clarify can-we-call-JIT logic
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 <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>

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