Make rc_for_type() pass Type arg by reference instead of value

[Firestar99]

This PR changes trait MachInst:

- fn rc_for_type(ty: Type) -> CodegenResult<(&'static [RegClass], &'static [Type])>;
+ fn rc_for_type(ty: &Type) -> CodegenResult<(&[RegClass], &[Type])>;

This change is trivial to support from all call sites, and allows implementors to:

- I8 => Ok((&[RegClass::Int], &[I8])),
- I16 => Ok((&[RegClass::Int], &[I16])),
- I32 => Ok((&[RegClass::Int], &[I32])),
- I64 => Ok((&[RegClass::Int], &[I64])),
- F32 => Ok((&[RegClass::Float], &[F32])),
- F64 => Ok((&[RegClass::Float], &[F64])),
+ I8 | I16 | I32 | I64 => Ok((&[RegClass::Int], core::slice::from_ref(ty))),
+ F32 | F64 => Ok((&[RegClass::Float], core::slice::from_ref(ty))),

Not sure how overall valuable this patch is. This function has just been annoying to implement for SSA-style IRs, where RegClass doesn't matter and it's just returning whatever type was passed.

[fitzgen]

This function has just been annoying to implement for SSA-style IRs, where RegClass doesn't matter and it's just returning whatever type was passed.

Can you clarify what you are doing here? Are you adding an SSA-style IR as a new backend to Cranelift?