pull out specialize_for_jit()

Not too tricky, but I did have to slightly work around
not having this be a member function of Builder.  Seems
healthy enough though, nothing terribly ugly or dangerous.

Still TODO, unit test specialize.  Probably another CL.

Change-Id: Ia547ae010940754fa7bb8417119416762597cb4c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/281045
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Herb Derby <herb@google.com>
2 files changed