LoongArch: testsuite:Added additional vectorization "-mlasx" compilation option.

In the LoongArch architecture, the reason for not adding the 128-bit
vector-width-*hi* instruction template in the GCC back end is that it causes
program performance loss, so we can only add the "-mlasx" compilation option
to use 256-bit vectorization functions in test files.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/bb-slp-pattern-1.c: If you are testing on the
	LoongArch architecture, you need to add the "-mlasx" compilation
	option to generate vectorized code.
	* gcc.dg/vect/slp-widen-mult-half.c: Dito.
	* gcc.dg/vect/vect-widen-mult-const-s16.c: Dito.
	* gcc.dg/vect/vect-widen-mult-const-u16.c: Dito.
	* gcc.dg/vect/vect-widen-mult-half-u8.c: Dito.
	* gcc.dg/vect/vect-widen-mult-half.c: Dito.
	* gcc.dg/vect/vect-widen-mult-u16.c: Dito.
	* gcc.dg/vect/vect-widen-mult-u8-s16-s32.c: Dito.
	* gcc.dg/vect/vect-widen-mult-u8-u32.c: Dito.
	* gcc.dg/vect/vect-widen-mult-u8.c: Dito.