[PATCH][AArch64] Implemented vmul/vmux intrinsics

[Tim Northover]

> The vmul_lane_f64 and the others mentioned below are legacy intrinsics with
> parameters of v1f64 type:
>
>     v1if64  vmul_lane_f64 (v1f64 a,  v1if64 v, i32 o)
>
> The compiler ends up getting rid of the vectors.

Ah, I see. So it's specifically that you want "vmul_lane_f64(x, y, 0)"
to produce "fmul d0, d0, v0.2[0]" rather than "fmul d0, d0, d0"?

If so, I don't think that's a property worth preserving. We were
forced into ugly scalar pseudo-vectors due to lack of global isel,
which doesn't apply here. I can't see any good reason to force the
compiler in this case: the instructions are completely
interchangeable.

Cheers.

Tim.