[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
https://reviews.llvm.org/D68408#1998112
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.

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