restore lost optimization when the shader can report const_in_y

accidentally lost we I landed https://codereview.chromium.org/1556003003

BUG=575905
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1576083002

Review URL: https://codereview.chromium.org/1576083002
1 file changed