Declare constants in the function preamble

This allows us to give names to constants in the constant pool and then use these names in the function body. The original behavior, specifiying the constant value as an instruction immediate, is still supported as a shortcut but some filetests had to change since the canonical way of printing the CLIF constants is now in the preamble.

cranelift/codegen/src/write.rs

@@ -102,6 +102,11 @@ pub trait FuncWriter {
             self.write_entity_definition(w, func, jt.into(), jt_data)?;
         }
 
+        for (&cref, cval) in func.dfg.constants.iter() {
+            any = true;
+            self.write_entity_definition(w, func, cref.into(), cval)?;
+        }
+
         Ok(any)
     }
 
@@ -494,6 +499,9 @@ pub fn write_operands(
         UnaryIeee64 { imm, .. } => write!(w, " {}", imm),
         UnaryBool { imm, .. } => write!(w, " {}", imm),
         UnaryGlobalValue { global_value, .. } => write!(w, " {}", global_value),
+        UnaryConst {
+            constant_handle, ..
+        } => write!(w, " {}", constant_handle),
         Binary { args, .. } => write!(w, " {}, {}", args[0], args[1]),
         BinaryImm { arg, imm, .. } => write!(w, " {}, {}", arg, imm),
         Ternary { args, .. } => write!(w, " {}, {}, {}", args[0], args[1], args[2]),
@@ -507,12 +515,6 @@ pub fn write_operands(
         NullAry { .. } => write!(w, " "),
         InsertLane { lane, args, .. } => write!(w, " {}, {}, {}", args[0], lane, args[1]),
         ExtractLane { lane, arg, .. } => write!(w, " {}, {}", arg, lane),
-        UnaryConst {
-            constant_handle, ..
-        } => {
-            let constant_data = dfg.constants.get(constant_handle);
-            write!(w, " {}", constant_data)
-        }
         Shuffle { mask, args, .. } => {
             let data = dfg.immediates.get(mask).expect(
                 "Expected the shuffle mask to already be inserted into the immediates table",