[InstCombine] Remove one-use limitation from X-Y==0 fold

This one-use limitation is artificial, we do not increase
instruction count if we perform the fold with multiple uses. The
motivating case is shown in @sub_eq_zero_select, where the one-use
limitation causes us to miss a subsequent select fold.

I believe the backend is pretty good about reusing flag-producing
subs for cmps with same operands, so I think doing this is fine.

Differential Revision: https://reviews.llvm.org/D120337