[PATCH][AArch64] Prefer ldp x, x to ldr q

[Tim Northover]

> While this is a slight fudge, I don't see it as a hack personally.

A "heuristic" then (scare-quotes optional)? That covers a multitude of sins.

Personally, I'd still say a hack, but that the entire function is a
hack once you start thinking about ldp so nothing else is possible
without reworking.

So I reckon the approach is OK for now, once it's made more generic
(i.e. it should probably return the same thing for all 128-bit ops).

Cheers.

Tim.