This is from D48754, use isKnownNegation() function to simplify more add instruction patterns. such as:

define i8 @knownnegation(i8 %x, i8 %y) { %xy = sub i8 %x, %y %yx = sub i8 %y, %x %r = add i8 %xy, %yx ret i8 %r }

--->

define i8 @knownnegation(i8 %x, i8 %y) { ret i8 0 }

Test cases are added at rL336822