[InstCombine] Negator: don't negate multi-use sub
While we can do that, it doesn't increase instruction count,
if the old sub sticks around then the transform is not only
not a unlikely win, but a likely regression, since we likely
now extended live range and use count of both of the sub operands,
as opposed to just the result of sub.
As Kostya Serebryany notes in post-commit review in
this indeed can degrade final assembly,
increase register pressure, and spilling.
This isn't what we want here,
so at least for now let's guard it with an use check.