[llvm] [RISCV] Don't custom lower direct fp_extends where possible (PR #125644)

[Luke Lau via llvm-commits]

lukel97 wrote:

> How much does this increase RISCVGenDAGISel.inc size. There should be a comment in the file like "Total Array size is [2365123](https://github.com/llvm/llvm-project/commit/2365123115f238add01e7d7d78654043d0440c55) bytes"

It goes from 2365123 bytes to 2472466 bytes, mostly because of the PatFrags. I'll see if there's a way to cut it down

https://github.com/llvm/llvm-project/pull/125644