[llvm] [RISCV] Remove {s,u}int_to_fp custom op action for f16/bf16. NFC (PR #111471)

[Luke Lau via llvm-commits]

lukel97 wrote:

Looks like the cost model tests are failing, so this isn't as NFC as I thought. BasicTTIImpl's getCastInstrCost isn't taking into account the swapped "legal" type either.

Since the affected test is for a type that will crash when lowering because it's > LMUL 8 and we don't split fixed length f16/bf16 vectors, I'm not too worried that the cost of it has increased.

https://github.com/llvm/llvm-project/pull/111471