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/meta/gen_legalizer.py

@@ -353,16 +353,12 @@ def gen_xform_group(xgrp, fmt, type_sets):
     fmt.doc_comment("Legalize the instruction pointed to by `pos`.")
     fmt.line('#[allow(unused_variables,unused_assignments)]')
     with fmt.indented(
-            'pub fn {}(dfg: &mut ir::DataFlowGraph, '
-            'cfg: &mut ::flowgraph::ControlFlowGraph, '
-            'pos: &mut ir::Cursor) -> '
+            'pub fn {}(inst: ir::Inst, '
+            'func: &mut ir::Function, '
+            'cfg: &mut ::flowgraph::ControlFlowGraph) -> '
             'bool {{'.format(xgrp.name), '}'):
         fmt.line('use ir::{InstBuilder, CursorBase};')
 
-        # Gen the instruction to be legalized. The cursor we're passed must be
-        # pointing at an instruction.
-        fmt.line('let inst = pos.current_inst().expect("need instruction");')
-
         # Group the xforms by opcode so we can generate a big switch.
         # Preserve ordering.
         xforms = defaultdict(list)  # type: DefaultDict[str, List[XForm]]
@@ -370,18 +366,23 @@ def gen_xform_group(xgrp, fmt, type_sets):
             inst = xform.src.rtl[0].expr.inst
             xforms[inst.camel_name].append(xform)
 
-        with fmt.indented('match dfg[inst].opcode() {', '}'):
-            for camel_name in sorted(xforms.keys()):
-                with fmt.indented(
-                        'ir::Opcode::{} => {{'.format(camel_name), '}'):
-                    for xform in xforms[camel_name]:
-                        gen_xform(xform, fmt, type_sets)
-            # We'll assume there are uncovered opcodes.
-            fmt.line('_ => {},')
+        with fmt.indented('{', '}'):
+            fmt.line(
+                    'let pos = &mut ir::Cursor::new(&mut func.layout)'
+                    '.at_inst(inst);')
+            fmt.line('let dfg = &mut func.dfg;')
+            with fmt.indented('match dfg[inst].opcode() {', '}'):
+                for camel_name in sorted(xforms.keys()):
+                    with fmt.indented(
+                            'ir::Opcode::{} => {{'.format(camel_name), '}'):
+                        for xform in xforms[camel_name]:
+                            gen_xform(xform, fmt, type_sets)
+                # We'll assume there are uncovered opcodes.
+                fmt.line('_ => {},')
 
         # If we fall through, nothing was expanded. Call the chain if any.
         if xgrp.chain:
-            fmt.format('{}(dfg, cfg, pos)', xgrp.chain.rust_name())
+            fmt.format('{}(inst, func, cfg)', xgrp.chain.rust_name())
         else:
             fmt.line('false')