[meta] Avoid unwrapping instructions several times during legalization;

This avoids doing multiple unpacking of the InstructionData for a single
legalization, improving readability and reducing size of the generated
code. For instance, icmp had to unpack the format once per IntCC
condition code.

cranelift/codegen/meta/src/cdsl/ast.rs

@@ -63,7 +63,7 @@ impl Def {
             format!("({})", results.join(", "))
         };
 
-        format!("{} << {}", results, self.apply.to_comment_string(var_pool))
+        format!("{} := {}", results, self.apply.to_comment_string(var_pool))
     }
 }