[llvm] [AArch64] Add costs for FP conversions with v3f32. (PR #107303)

[Florian Hahn via llvm-commits]

fhahn wrote:

> This should work by finding the legal types. It isn't going to scale very well if we have to add every individual type size.

That's a good point, thanks! Updated the PR to try to do this in a more general way, which required to do it in BasicTTImpl.h directly.  A number of costs changed for X86 and ARM as well. Glanced at them briefly and they roughly seem to make sense (very high cost reduced to slightly lower cost in most cases).

There's already logic to handle the case where vectors need to be split in BasicTTI. The current logic should eventually replace that logic as well, but at the moment this would lead to more test changes 

https://github.com/llvm/llvm-project/pull/107303