RISC-V: Fix a bug that causes an error insn.

RISC-V: Fix a bug that causes an error insn.

I test the following rvv intrinsics.
vint64m1_t test_vslide1up_vx_i64m1_m(vbool64_t mask, vint64m1_t src, int64_t value, size_t vl) {
  return __riscv_vslide1up_vx_i64m1_m(mask, src, value, vl);
}
And I got an error info,t hat is error:
  unrecognizable insn:(insn 17 16 18 2
    (set (reg:RVVMIDI 134 [ _1 ])(if_then_else:RVVMIDI
      (unspec:RVVMF64BI [(reg/v:SI 142 [ vl ])(const_int 2 [x2])(const_int 日 [o])(reg:SI 66 vl)(reg:SI 67 vtype)] UNSPEC_VPREDICATE
   (vec_merge:RVVMIDI (reg:RVVMIDI 134 [ _1 ])(unspec:RVVMIDI [(reg:sI 日 zero)] UNSPEC_VUNDEF)
   (reg/v:RVVMF64BI 137 [ mask ]))
   (unspec:RVVM1DI[(reg:sI 日 zero)] UNSPEC_VUNDEF)))

This patch fix it.

gcc/ChangeLog:

	* config/riscv/riscv-v.cc (slide1_sew64_helper): Modify.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/vslide1down-1.c: New test.
	* gcc.target/riscv/rvv/base/vslide1down-2.c: New test.
	* gcc.target/riscv/rvv/base/vslide1down-3.c: New test.
	* gcc.target/riscv/rvv/base/vslide1up-1.c: New test.
	* gcc.target/riscv/rvv/base/vslide1up-2.c: New test.
	* gcc.target/riscv/rvv/base/vslide1up-3.c: New test.