Revisit expanded instructions for legalization.

When an illegal instruction is replaced with other instructions, back up
and revisit the expanded instructions. The new instructions need to have
encodings assigned too.

This also allows for expansions to contain illegal instructions that
need to be legalized themselves.

lib/cretonne/meta/gen_legalizer.py

@@ -139,8 +139,6 @@ def gen_xform(xform, fmt):
     `inst: Inst` is the variable to be replaced. It is pointed to by `pos:
     Cursor`.
     `dfg: DataFlowGraph` is available and mutable.
-
-    Produce an `Option<Inst>` result at the end.
     """
     # Unwrap the source instruction, create local variables for the input
     # variables.
@@ -150,9 +148,6 @@ def gen_xform(xform, fmt):
     for dst in xform.dst.rtl:
         emit_dst_inst(dst, fmt)
 
-    # TODO: Return the first replacement instruction.
-    fmt.line('None')
-
 
 def gen_xform_group(xgrp, fmt):
     # type: (XFormGroup, Formatter) -> None
@@ -165,8 +160,7 @@ def gen_xform_group(xgrp, fmt):
     fmt.line('#[allow(unused_variables,unused_assignments)]')
     with fmt.indented(
             'fn ' + xgrp.name +
-            '(pos: &mut Cursor, dfg: &mut DataFlowGraph) -> ' +
-            'Option<Inst> {',
+            '(pos: &mut Cursor, dfg: &mut DataFlowGraph) -> bool {',
             '}'):
         # Gen the instruction to be legalized. The cursor we're passed must be
         # pointing at an instruction.
@@ -179,7 +173,8 @@ def gen_xform_group(xgrp, fmt):
                         'Opcode::{} => {{'.format(inst.camel_name), '}'):
                     gen_xform(xform, fmt)
             # We'll assume there are uncovered opcodes.
-            fmt.line('_ => None,')
+            fmt.line('_ => return false,')
+        fmt.line('true')
 
 
 def generate(isas, out_dir):