Improved gradient table generation performance for two-stop gradients.

Improved gradient table generation performance for two-stop gradients.

Two stops is a fairly common case so we gain quite a bit by special
casing it. Improves performance by 10 % in parcycle benchmark, and
by 90 % in a synthetic benchmark.

Reviewed-by: Andreas Kling
(cherry picked from commit 5b74a70ac630073582be56f8a0539624a1080185)