RISC-V: Robostify dynamic lmul test

While working on refining the cost model, I notice this test will generate unexpected
scalar xor instructions if we don't tune cost model carefully.

Add more assembler to avoid future regression.

Committed.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul2-7.c: Add assembler-not check.

gcc/testsuite/gcc.dg/vect/costmodel/riscv/rvv/dynamic-lmul2-7.c

@@ -22,3 +22,4 @@ x264_pixel_8x8 (unsigned char *pix1, unsigned char *pix2, int i_stride_pix2)
 }
 
 /* { dg-final { scan-assembler {e32,m2} } } */
+/* { dg-final { scan-assembler-not {xor} } } */