LoongArch: Implement vec_widen_mult_{even,odd}_* for LSX and LASX modes

Since PR116142 has been fixed, now we can add the standard names so the
compiler will generate better code if the result of a widening
production is reduced.

gcc/ChangeLog:

	* config/loongarch/simd.md (even_odd): New define_int_attr.
	(vec_widen_<su>mult_<even_odd>_<mode>): New define_expand.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/wide-mul-reduc-1.c: New test.
	* gcc.target/loongarch/wide-mul-reduc-2.c: New test.