re PR target/79079 ([6] Wrong code gen for __builtin_mul_overflow when TRULY_NOOP_TRUNCATION (32, 64) == false)

	PR target/79079
	* internal-fn.c (expand_mul_overflow): Use convert_modes instead of
	gen_lowpart.

Co-Authored-By: Kuan-Lin Chen <kuanlinchentw@gmail.com>

From-SVN: r244539