Fix pretty print of atomic_rmw clif ops (#4441)

A comma was accidentally placed after the operation itself when no comma
is necessary (only after operands, not the operation itself).

cranelift/codegen/src/write.rs

@@ -380,7 +380,7 @@ pub fn write_operands(w: &mut dyn Write, dfg: &DataFlowGraph, inst: Inst) -> fmt
     let pool = &dfg.value_lists;
     use crate::ir::instructions::InstructionData::*;
     match dfg[inst] {
-        AtomicRmw { op, args, .. } => write!(w, " {}, {}, {}", op, args[0], args[1]),
+        AtomicRmw { op, args, .. } => write!(w, " {} {}, {}", op, args[0], args[1]),
         AtomicCas { args, .. } => write!(w, " {}, {}, {}", args[0], args[1], args[2]),
         LoadNoOffset { flags, arg, .. } => write!(w, "{} {}", flags, arg),
         StoreNoOffset { flags, args, .. } => write!(w, "{} {}, {}", flags, args[0], args[1]),