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/src/isa/intel/mod.rs

@@ -72,6 +72,7 @@ impl TargetIsa for Isa {
                        self.cpumode,
                        &enc_tables::LEVEL2[..],
                        &enc_tables::ENCLISTS[..],
+                       &enc_tables::LEGALIZE_ACTIONS[..],
                        &enc_tables::RECIPE_PREDICATES[..],
                        &enc_tables::INST_PREDICATES[..],
                        self.isa_flags.predicate_view())