slp: support complex multiply and complex multiply conjugate

This adds support for complex multiply and complex multiply and accumulate to
the vect pattern detector.

Example of instructions matched:

#include <stdio.h>
#include <complex.h>

#define N 200
#define ROT
#define TYPE float
#define TYPE2 float

void g (TYPE2 complex a[restrict N], TYPE complex b[restrict N], TYPE complex c[restrict N])
{
  for (int i=0; i < N; i++)
    {
      c[i] =  a[i] * (b[i] ROT);
    }
}

void g_f1 (TYPE2 complex a[restrict N], TYPE complex b[restrict N], TYPE complex c[restrict N])
{
  for (int i=0; i < N; i++)
    {
      c[i] =  conjf (a[i]) * (b[i] ROT);
    }
}

void g_s1 (TYPE2 complex a[restrict N], TYPE complex b[restrict N], TYPE complex c[restrict N])
{
  for (int i=0; i < N; i++)
    {
      c[i] =  a[i] * conjf (b[i] ROT);
    }
}

gcc/ChangeLog:

	* internal-fn.def (COMPLEX_MUL, COMPLEX_MUL_CONJ): New.
	* optabs.def (cmul_optab, cmul_conj_optab): New.
	* doc/md.texi: Document them.
	* tree-vect-slp-patterns.c (vect_match_call_complex_mla,
	vect_normalize_conj_loc, is_eq_or_top, vect_validate_multiplication,
	vect_build_combine_node, class complex_mul_pattern,
	complex_mul_pattern::matches, complex_mul_pattern::recognize,
	complex_mul_pattern::build): New.