[llvm] [RISCV][MC] Recognise that fcvt.d.s with frm != 0b000 is valid (PR #67555)

[Craig Topper via llvm-commits]

topperc wrote:

Do we end up printing the `rne` with no-aliases now? The check lines don't check the whole line so they wouldn't fail if an extra operand started printing.

Would it be better to have new versions of these for these instruction so we can specific a different default function?

```
def FRMArg : AsmOperandClass {                                                   
  let Name = "FRMArg";                                                           
  let RenderMethod = "addFRMArgOperands";                                        
  let ParserMethod = "parseFRMArg";                                              
  let IsOptional = 1;                                                            
  let DefaultMethod = "defaultFRMArgOp";                                         
}                                                                                
                                                                                 
def frmarg : Operand<XLenVT> {                                                   
  let ParserMatchClass = FRMArg;                                                 
  let PrintMethod = "printFRMArg";                                               
  let DecoderMethod = "decodeFRMArg";                                            
} 
```

https://github.com/llvm/llvm-project/pull/67555