x86/Intel: correct VFPCLASSP{S,D} handling when displacement is present

fits_in_disp8() can be called before ambiguous operands get resolved
or rejected (in process_suffix()), which requires that i.memshift be
non-negative to avoid an internal error. This case wasn't covered by
6c0946d0 ("x86: correct VFPCLASSP{S,D} operand size handling").