RISC-V: Fix asm check failure for truncated after SAT_SUB

It seems that the asm check is incorrect for truncated after SAT_SUB,
we should take the vx check for vssubu instead of vv check.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub_trunc-1.c:
	Update vssubu check from vv to vx.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub_trunc-2.c:
	Ditto.
	* gcc.target/riscv/rvv/autovec/binop/vec_sat_u_sub_trunc-3.c:
	Ditto.

Signed-off-by: Pan Li <pan2.li@intel.com>