[AArch64] Unify vec_set patterns, support floating-point vector modes properly

[AArch64] Unify vec_set patterns, support floating-point vector modes properly

We've a deficiency in our vec_set family of patterns.
We don't support directly loading a vector lane using LD1 for V2DImode and all the vector floating-point modes.
We do do it correctly for the other integer vector modes (V4SI, V8HI etc) though.

The alternatives on the relative floating-point patterns only allow a register-to-register INS instruction.
That means if we want to load a value into a vector lane we must first load it into a scalar register and then
perform an INS, which is wasteful.

There is also an explicit V2DI vec_set expander dangling around for no reason that I can see. It seems to do the
exact same things as the other vec_set expanders. This patch removes that.
It now unifies all vec_set expansions into a single "vec_set<mode>" define_expand using the catch-all VALL_F16 iterator.

With this patch we avoid loading values into scalar registers and then doing an explicit INS on them to move them into
the desired vector lanes. For example for:

typedef float v4sf __attribute__ ((vector_size (16)));
typedef long long v2di __attribute__ ((vector_size (16)));

v2di
foo_v2di (long long *a, long long *b)
{
  v2di res = { *a, *b };
  return res;
}

v4sf
foo_v4sf (float *a, float *b, float *c, float *d)
{
  v4sf res = { *a, *b, *c, *d };
  return res;
}

we currently generate:

foo_v2di:
        ldr     d0, [x0]
        ldr     x0, [x1]
        ins     v0.d[1], x0
        ret

foo_v4sf:
        ldr     s0, [x0]
        ldr     s3, [x1]
        ldr     s2, [x2]
        ldr     s1, [x3]
        ins     v0.s[1], v3.s[0]
        ins     v0.s[2], v2.s[0]
        ins     v0.s[3], v1.s[0]
        ret

but with this patch we generate the much cleaner:
foo_v2di:
        ldr     d0, [x0]
        ld1     {v0.d}[1], [x1]
        ret

foo_v4sf:
        ldr     s0, [x0]
        ld1     {v0.s}[1], [x1]
        ld1     {v0.s}[2], [x2]
        ld1     {v0.s}[3], [x3]
        ret

	* config/aarch64/aarch64-simd.md (vec_set<mode>): Use VALL_F16 mode
	iterator.  Delete separate integer-mode vec_set<mode> expander.
	(aarch64_simd_vec_setv2di): Delete.
	(vec_setv2di): Delete.
	(aarch64_simd_vec_set<mode>): Delete all other patterns with that name.
	Use VALL_F16 mode iterator.  Add LD1 alternative and use vwcore for
	the "w, r" alternative.

	* gcc.target/aarch64/vect-init-ld1.c: New test.

From-SVN: r260351