autovectorizer: Test autovectorization of different dot-prod modes.HEADtrunkmaster

Given the novel treatment of the dot product optab as a conversion, we
are now able to target different relationships between output modes and
input modes.

This is made clearer by way of example. Previously, on AArch64, the
following loop was vectorizable:

uint32_t udot4(int n, uint8_t* data) {
  uint32_t sum = 0;
  for (int i=0; i<n; i+=1)
    sum += data[i] * data[i];
  return sum;
}

while the following was not:

uint32_t udot2(int n, uint16_t* data) {
  uint32_t sum = 0;
  for (int i=0; i<n; i+=1)
    sum += data[i] * data[i];
  return sum;
}

Under the new treatment of the dot product optab, they are both now
vectorizable.

This adds the relevant target-agnostic check to ensure this behavior
in the autovectorizer, gated behind the new check_effective_target
`vect_dotprod_hisi' as well a runtime check targeting aarch64.

gcc/testsuite/ChangeLog:

	* lib/target-supports.exp (check_effective_target_vect_dotprod_hisi):
	New.
	* gcc.dg/vect/vect-dotprod-conv-optab.c: Likewise.
	* gcc.target/aarch64/vect-dotprod-twoway-hisi.c: Likewise.

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