Make functions cloneable for testing.

It's not super fast to clone a function, but it is faster than
re-parsing the test case file it came from.

Some tests want to mutate the function, and there may be other tests in
the same script that need the original function.

src/libcretonne/ir/dfg.rs

@@ -15,6 +15,7 @@ use std::u16;
 /// The layout of EBBs in the function and of instructions in each EBB is recorded by the
 /// `FunctionLayout` data structure which form the other half of the function representation.
 ///
+#[derive(Clone)]
 pub struct DataFlowGraph {
     /// Data about all of the instructions in the function, including opcodes and operands.
     /// The instructions in this map are not in program order. That is tracked by `Layout`, along
@@ -119,6 +120,7 @@ pub enum ValueDef {
 }
 
 // Internal table storage for extended values.
+#[derive(Clone)]
 enum ValueData {
     // Value is defined by an instruction, but it is not the first result.
     Inst {
@@ -336,6 +338,7 @@ impl DataFlowGraph {
 // Arguments for an extended basic block are values that dominate everything in the EBB. All
 // branches to this EBB must provide matching arguments, and the arguments to the entry EBB must
 // match the function arguments.
+#[derive(Clone)]
 struct EbbData {
     // First argument to this EBB, or `NO_VALUE` if the block has no arguments.
     //