[llvm] [RISCV] Transform fcmp to is.fpclass (PR #120242)

[Gergely Futo via llvm-commits]

futog wrote:

> FYI I did similar transformation in CodeGenPrepare: #81572.

In CodeGenPrepare it is only for `fcInf` || `fcInf | fcNan` right? Aren't we considering the rest of the classifications on purpose?

https://github.com/llvm/llvm-project/pull/120242