AArch64: Implement widen_[us]sum using [US]ADDW[TB] for SVE2 [PR122069]

AArch64: Implement widen_[us]sum using [US]ADDW[TB] for SVE2 [PR122069]

SVE2 adds [US]ADDW[TB] which we can use when we have to do a single step
widening addition.  This is useful for instance when the value to be widened
does not come from a load.  For example for

int foo2_int(unsigned short *x, unsigned short * restrict y) {
  int sum = 0;
  for (int i = 0; i < 8000; i++)
    {
      x[i] = x[i] + y[i];
      sum += x[i];
    }
  return sum;
}

we used to generate

.L6:
        ld1h    z1.h, p7/z, [x0, x2, lsl 1]
        ld1h    z29.h, p7/z, [x1, x2, lsl 1]
        add     z29.h, z29.h, z1.h
        punpklo p6.h, p7.b
        uunpklo z0.s, z29.h
        add     z31.s, p6/m, z31.s, z0.s
        punpkhi p6.h, p7.b
        uunpkhi z30.s, z29.h
        add     z31.s, p6/m, z31.s, z30.s
        st1h    z29.h, p7, [x0, x2, lsl 1]
        add     x2, x2, x4
        whilelo p7.h, w2, w3
        b.any   .L6
        ptrue   p7.b, all
        uaddv   d31, p7, z31.s

but with +sve2

.L12:
        ld1h    z30.h, p7/z, [x0, x2, lsl 1]
        ld1h    z29.h, p7/z, [x1, x2, lsl 1]
        add     z30.h, z30.h, z29.h
        uaddwb  z31.s, z31.s, z30.h
        uaddwt  z31.s, z31.s, z30.h
        st1h    z30.h, p7, [x0, x2, lsl 1]
        mov     x3, x2
        inch    x2
        cmp     w2, w4
        bls     .L12
        inch    x3
        uaddv   d31, p7, z31.s

gcc/ChangeLog:

	PR middle-end/122069
	* config/aarch64/aarch64-sve2.md: (widen_ssum<mode><Vnarrow>3): New.
	(widen_usum<mode><Vnarrow>3): New.
	* config/aarch64/iterators.md (Vnarrow): New, to match VNARROW.

gcc/testsuite/ChangeLog:

	PR middle-end/122069
	* gcc.target/aarch64/sve2/pr122069_1.c: New test.
	* gcc.target/aarch64/sve2/pr122069_2.c: New test.