LoongArch: testsuite:Fix FAIL in lasx-xvstelm.c file.

After implementing the cost model on the LoongArch architecture, the GCC
compiler code has this feature turned on by default, which causes the
lasx-xvstelm.c file test to fail. Through analysis, this test case can
generate vectorization instructions required for detection only after
disabling the functionality of the cost model with the "-fno-vect-cost-model"
compilation option.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vector/lasx/lasx-xvstelm.c:Add compile
	option "-fno-vect-cost-model" to dg-options.