x64: add support for packed promote and demote (#2783)

* Add support for x64 packed promote low

* Add support for x64 packed floating point demote

* Update vector promote low and demote by adding constraints

Also does some renaming and minor refactoring

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -3193,6 +3193,8 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
 
         Opcode::TlsValue => unimplemented!("tls_value"),
         Opcode::FcvtLowFromSint => unimplemented!("FcvtLowFromSint"),
+        Opcode::FvpromoteLow => unimplemented!("FvpromoteLow"),
+        Opcode::Fvdemote => unimplemented!("Fvdemote"),
     }
 
     Ok(())