Add a Function::clear() method.

This makes it possible to clear out a Function data structure so it can
be reused for compiling multiple functions.

Also add clear() methods to various sub-structures.

lib/cretonne/src/ir/dfg.rs

@@ -74,6 +74,17 @@ impl DataFlowGraph {
         }
     }
 
+    /// Clear everything.
+    pub fn clear(&mut self) {
+        self.insts.clear();
+        self.results.clear();
+        self.ebbs.clear();
+        self.value_lists.clear();
+        self.values.clear();
+        self.signatures.clear();
+        self.ext_funcs.clear();
+    }
+
     /// Get the total number of instructions created in this function, whether they are currently
     /// inserted in the layout or not.
     ///

lib/cretonne/src/ir/extfunc.rs

@@ -47,6 +47,14 @@ impl Signature {
         }
     }
 
+    /// Clear the signature so it is identical to a fresh one returned by `new()`.
+    pub fn clear(&mut self, call_conv: CallConv) {
+        self.argument_types.clear();
+        self.return_types.clear();
+        self.call_conv = call_conv;
+        self.argument_bytes = None;
+    }
+
     /// Compute the size of the stack arguments and mark signature as legalized.
     ///
     /// Even if there are no stack arguments, this will set `argument_types` to `Some(0)` instead

lib/cretonne/src/ir/function.rs

@@ -74,6 +74,20 @@ impl Function {
         }
     }
 
+    /// Clear all data structures in this function.
+    pub fn clear(&mut self) {
+        self.signature.clear(ir::CallConv::Native);
+        self.stack_slots.clear();
+        self.global_vars.clear();
+        self.heaps.clear();
+        self.jump_tables.clear();
+        self.dfg.clear();
+        self.layout.clear();
+        self.encodings.clear();
+        self.locations.clear();
+        self.offsets.clear();
+    }
+
     /// Create a new empty, anonymous function with a native calling convention.
     pub fn new() -> Function {
         Self::with_name_signature(FunctionName::default(), Signature::new(CallConv::Native))

lib/cretonne/src/ir/layout.rs

@@ -51,6 +51,14 @@ impl Layout {
             last_ebb: None,
         }
     }
+
+    /// Clear the layout.
+    pub fn clear(&mut self) {
+        self.ebbs.clear();
+        self.insts.clear();
+        self.first_ebb = None;
+        self.last_ebb = None;
+    }
 }
 
 // Sequence numbers.