test: Fix XPASS of bb-slp-43.c for RVV

RVV is variable length vector but also has 256 bit VLS mode vector.
This test is vectorized as:

f:
        vsetivli        zero,8,e32,m2,ta,ma
        vle32.v v2,0(a0)
        vmv.v.i v4,1
        vle16.v v1,0(a1)
        vmseq.vv        v0,v2,v4
        vsetvli zero,zero,e16,m1,ta,ma
        vmseq.vi        v1,v1,2
        vsetvli zero,zero,e32,m2,ta,ma
        vmv.v.i v2,0
        vmand.mm        v0,v0,v1
        vmerge.vvm      v2,v2,v4,v0
        vse32.v v2,0(a0)
        ret

Use 256 bit vector, so remove XFAIL for 256 bits vector.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/bb-slp-43.c: Fix XPASS for RVV.