[DAGCombiner] Don't expand (neg (abs x)) if the abs has an additional user.

If the types aren't legal, the expansions may get type legalized in a
different way preventing code sharing. If the type is legal, we will
share some instructions between the two expansions, but we will need an
extra register.

Since we don't appear to fold (neg (sub A, B)) if the sub has an
additional user, I think it makes sense not to expand NABS.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D120513