If you have an intrinsic that doesn't have a 1:1 mapping with an instruction, and the types passed into that intrinsic aren't fully defined by their register banks, GISel can't actually map it to the right instruction today. As far as I know, addp is the only such intrinsic in AArch64.
Introduce a hacky workaround in the AArch64 legalizer to make sure we always fall back on this intrinsic. Goal here is to avoid impacting other backends as much as possible while we work towards a proper solution for the problem.
Also loosen some constraints on the legality of instructions with custom legalization, and factor out an assert. Before, we assumed that legalization would always *modify* an instruction *into* a legal state (or it'd be listed as legal based off the input LLTs). Since LLTs aren't sufficient here, this assumption falls over.
Also, we had the assumption that we'd be passing along LLTs into apply(), which isn't true here either.