match.pd: Optimize __builtin_mul_overflow_p (x, cst, (utype)0) to x > ~(utype)0 / cst [PR30314]

A comparison with a constant is most likely always faster than
.MUL_OVERFLOW from which we only check whether it overflowed and not the
multiplication result, and even if not, it is simpler operation on GIMPLE
and even if a target exists where such multiplications with overflow checking
are cheaper than comparisons, because comparisons are so much more common
than overflow checking multiplications, it would be nice if it simply
arranged for comparisons to be emitted like those multiplications on its
own...

2022-06-01  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/30314
	* match.pd (__builtin_mul_overflow_p (x, cst, (utype) 0) ->
	x > ~(utype)0 / cst): New simplification.

	* gcc.dg/tree-ssa/pr30314.c: New test.