AArch64: Add NEON, SVE and SVE2 RTL patterns for Complex Addition.

This adds implementation for the optabs for add complex operations.  With this
the following C code:

  void f90 (float complex a[restrict N], float complex b[restrict N],
	    float complex c[restrict N])
  {
    for (int i=0; i < N; i++)
      c[i] = a[i] + (b[i] * I);
  }

generates

  f90:
	  mov     x3, 0
	  .p2align 3,,7
  .L2:
	  ldr     q0, [x0, x3]
	  ldr     q1, [x1, x3]
	  fcadd   v0.4s, v0.4s, v1.4s, #90
	  str     q0, [x2, x3]
	  add     x3, x3, 16
	  cmp     x3, 1600
	  bne     .L2
	  ret

instead of

  f90:
	  add     x3, x1, 1600
	  .p2align 3,,7
  .L2:
	  ld2     {v4.4s - v5.4s}, [x0], 32
	  ld2     {v2.4s - v3.4s}, [x1], 32
	  fsub    v0.4s, v4.4s, v3.4s
	  fadd    v1.4s, v5.4s, v2.4s
	  st2     {v0.4s - v1.4s}, [x2], 32
	  cmp     x3, x1
	  bne     .L2
	  ret

gcc/ChangeLog:

	* config/aarch64/aarch64-simd.md (cadd<rot><mode>3): New.
	* config/aarch64/iterators.md (SVE2_INT_CADD_OP): New.
	* config/aarch64/aarch64-sve.md (cadd<rot><mode>3): New.
	* config/aarch64/aarch64-sve2.md (cadd<rot><mode>3): New.