RISC-V: Allow RVV VMS{Compare}(V1, V1) simplify to VMCLR

When some RVV integer compare operators act on the same vector
registers without mask. They can be simplified to VMCLR.

This PATCH allow the ne, lt, ltu, gt, gtu to perform such kind
of the simplification by adding one new define_split.

Given we have:
vbool1_t test_shortcut_for_riscv_vmslt_case_0(vint8m8_t v1, size_t vl) {
  return __riscv_vmslt_vv_i8m8_b1(v1, v1, vl);
}

Before this patch:
vsetvli  zero,a2,e8,m8,ta,ma
vl8re8.v v24,0(a1)
vmslt.vv v8,v24,v24
vsetvli  a5,zero,e8,m8,ta,ma
vsm.v    v8,0(a0)
ret

After this patch:
vsetvli zero,a2,e8,mf8,ta,ma
vmclr.m v24                    <- optimized to vmclr.m
vsetvli zero,a5,e8,mf8,ta,ma
vsm.v   v24,0(a0)
ret

As above, we may have one instruction eliminated and require less
vector registers.

gcc/ChangeLog:

	* config/riscv/vector.md: Add new define split to perform
	the simplification.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/integer_compare_insn_shortcut.c: New test.

Signed-off-by: Pan Li <pan2.li@intel.com>
Co-authored-by: kito-cheng <kito.cheng@sifive.com>