aarch64: Add and use FLAG_LOAD in builtins

aarch64: Add and use FLAG_LOAD in builtins

We already have a STORE flag that we use for builtins. This patch introduces a LOAD set
that uses AUTO_FP and FLAG_READ_MEMORY. This allows for more aggressive optimisation of the load
intrinsics.

Turns out we have a great many testcases that do:
float16x4x2_t
f_vld2_lane_f16 (float16_t * p, float16x4x2_t v)
{
  float16x4x2_t res;
  /* { dg-error "lane 4 out of range 0 - 3" "" { target *-*-* } 0 } */
  res = vld2_lane_f16 (p, v, 4);
  /* { dg-error "lane -1 out of range 0 - 3" "" { target *-*-* } 0 } */
  res = vld2_lane_f16 (p, v, -1);
  return res;
}

but since the first res is unused it now gets eliminated early on before we get to give an error
message. Ideally we'd like to warn for both.
This patch takes the conservative approach and doesn't convert the load-lane builtins to LOAD ;
that's something we can improve later.

gcc/ChangeLog:

	* config/aarch64/aarch64-builtins.c (FLAG_LOAD): Define.
	* config/aarch64/aarch64-simd-builtins.def (ld1x2, ld2, ld3, ld4, ld2r,
	ld3r, ld4r, ld1, ld1x3, ld1x4): Use LOAD flags.