RISC-V: Support widening register overlap for vf4/vf8

RISC-V: Support widening register overlap for vf4/vf8

size_t
foo (char const *buf, size_t len)
{
  size_t sum = 0;
  size_t vl = __riscv_vsetvlmax_e8m8 ();
  size_t step = vl * 4;
  const char *it = buf, *end = buf + len;
  for (; it + step <= end;)
    {
      vint8m1_t v0 = __riscv_vle8_v_i8m1 ((void *) it, vl);
      it += vl;
      vint8m1_t v1 = __riscv_vle8_v_i8m1 ((void *) it, vl);
      it += vl;
      vint8m1_t v2 = __riscv_vle8_v_i8m1 ((void *) it, vl);
      it += vl;
      vint8m1_t v3 = __riscv_vle8_v_i8m1 ((void *) it, vl);
      it += vl;

      asm volatile("nop" ::: "memory");
      vint64m8_t vw0 = __riscv_vsext_vf8_i64m8 (v0, vl);
      vint64m8_t vw1 = __riscv_vsext_vf8_i64m8 (v1, vl);
      vint64m8_t vw2 = __riscv_vsext_vf8_i64m8 (v2, vl);
      vint64m8_t vw3 = __riscv_vsext_vf8_i64m8 (v3, vl);

      asm volatile("nop" ::: "memory");
      size_t sum0 = __riscv_vmv_x_s_i64m8_i64 (vw0);
      size_t sum1 = __riscv_vmv_x_s_i64m8_i64 (vw1);
      size_t sum2 = __riscv_vmv_x_s_i64m8_i64 (vw2);
      size_t sum3 = __riscv_vmv_x_s_i64m8_i64 (vw3);

      sum += sumation (sum0, sum1, sum2, sum3);
    }
  return sum;
}

Before this patch:

        add     a3,s0,s1
        add     a4,s6,s1
        add     a5,s7,s1
        vsetvli zero,s0,e64,m8,ta,ma
        vle8.v  v4,0(s1)
        vle8.v  v3,0(a3)
        mv      s1,s2
        vle8.v  v2,0(a4)
        vle8.v  v1,0(a5)
        nop
        vsext.vf8       v8,v4
        vsext.vf8       v16,v2
        vs8r.v  v8,0(sp)
        vsext.vf8       v24,v1
        vsext.vf8       v8,v3
        nop
        vmv.x.s a1,v8
        vl8re64.v       v8,0(sp)
        vmv.x.s a3,v24
        vmv.x.s a2,v16
        vmv.x.s a0,v8
        add     s2,s2,s5
        call    sumation
        add     s3,s3,a0
        bgeu    s4,s2,.L5

After this patch:

	add	a3,s0,s1
	add	a4,s6,s1
	add	a5,s7,s1
	vsetvli	zero,s0,e64,m8,ta,ma
	vle8.v	v15,0(s1)
	vle8.v	v23,0(a3)
	mv	s1,s2
	vle8.v	v31,0(a4)
	vle8.v	v7,0(a5)
	vsext.vf8	v8,v15
	vsext.vf8	v16,v23
	vsext.vf8	v24,v31
	vsext.vf8	v0,v7
	vmv.x.s	a3,v0
	vmv.x.s	a2,v24
	vmv.x.s	a1,v16
	vmv.x.s	a0,v8
	add	s2,s2,s5
	call	sumation
	add	s3,s3,a0
	bgeu	s4,s2,.L5

	PR target/112431

gcc/ChangeLog:

	* config/riscv/vector.md: Add widening overlap of vf2/vf4.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/pr112431-16.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-17.c: New test.
	* gcc.target/riscv/rvv/base/pr112431-18.c: New test.