Switch to a FuncCursor in the top-level legalizer loop.

Stop passing Cursor references to legalizer functions. Give them the
whole &mut Function instead. Given the whole Function reference, these
functions can create their own cursors.

This lets legalizer actions access other Function data structures like
the global variables.

lib/cretonne/src/isa/mod.rs

@@ -138,9 +138,9 @@ impl settings::Configurable for Builder {
 /// legalize it?
 ///
 /// The `Encodings` iterator returns a legalization function to call.
-pub type Legalize = fn(&mut ir::DataFlowGraph,
-                       &mut flowgraph::ControlFlowGraph,
-                       &mut ir::Cursor)
+pub type Legalize = fn(ir::Inst,
+                       &mut ir::Function,
+                       &mut flowgraph::ControlFlowGraph)
                        -> bool;
 
 /// Methods that are specialized to a target ISA.