correct only spill once

In the scaler code, hoisted stack positions worked by chance. Add asserts,
and properly account for hoisted stack positions.

Change-Id: I8953d9f34dc110c7eadfd8855a02786aa6789043
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/285101
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
1 file changed