[InstCombine] Negator: 0 - (X + Y) --> (-X) - Y iff a single operand negated

This was the most obvious regression in
f5df5cd5.f5df5cd5

We really don't want to do this if the original/outermost subtraction
isn't a negation, and therefore doesn't go away - just sinking negation
isn't a win. We are actually appear to be missing folds so hoist it.

https://rise4fun.com/Alive/tiVe