[ConstraintElimination] Use simplified constraint for == 0.

When checking x == 0, checking x u<= 0 is sufficient and simpler than
x u>= 0 && x u<= 0.

https://alive2.llvm.org/ce/z/btM7d3

    ----------------------------------------
    define i1 @src(i4 %a) {
    %0:
      %c = icmp eq i4 %a, 0
      ret i1 %c
    }
    =>
    define i1 @tgt(i4 %a) {
    %0:
       %c = icmp ule i4 %a, 0
       ret i1 %c
    }
    Transformation seems to be correct!