LoongArch: Remove the "-mlasx" compilation option from the test cases.

LoongArch: Remove the "-mlasx" compilation option from the test cases.

Because LoongArch previously lacked support for 128-bit vectors
(vec_widen_{add/sub/mult}_{hi/lo}), the following test cases
needed to be enabled for 256-bit vectors to pass.
Now that 128-bit support has been added, "-mlasx" has been removed.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/slp-widen-mult-half.c: Remove '-mlasx'.
	* gcc.dg/vect/tree-vect.h: Check whether the runtime
	environment supports LSX instructions.
	* 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.