Return a function pointer from TargetIsa::encode().

Replace the isa::Legalize enumeration with a function pointer. This
allows an ISA to define its own specific legalization actions instead of
relying on the default two.

Generate a LEGALIZE_ACTIONS table for each ISA which contains
legalization function pointers indexed by the legalization codes that
are already in the encoding tables. Include this table in
isa/*/enc_tables.rs.

Give the `Encodings` iterator a reference to the action table and change
its `legalize()` method to return a function pointer instead of an
ISA-specific code.

The Result<> returned from TargetIsa::encode() no longer implements
Debug, so eliminate uses of unwrap and expect on that type.

lib/cretonne/meta/gen_instr.py

@@ -345,6 +345,8 @@ def gen_typesets_table(fmt, type_sets):
     """
     Generate the table of ValueTypeSets described by type_sets.
     """
+    if len(type_sets.table) == 0:
+        return
     fmt.comment('Table of value type sets.')
     assert len(type_sets.table) <= typeset_limit, "Too many type sets"
     with fmt.indented(