bpo-46504: faster code for trial quotient in x_divrem() (GH-30856)

* bpo-46504: faster code for trial quotient in x_divrem()

This brings x_divrem() back into synch with x_divrem1(), which was changed
in bpo-46406 to generate faster code to find machine-word division
quotients and remainders. Modern processors compute both with a single
machine instruction, but convincing C to exploit that requires writing
_less_ "clever" C code.