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/regalloc/spilling.rs

@@ -533,10 +533,10 @@ impl<'a> Context<'a> {
         let ty = dfg.value_type(copy);
 
         // Give it an encoding.
-        let encoding = self.isa
-            .encode(dfg, &dfg[inst], ty)
-            .expect("Can't encode copy");
-        *self.encodings.ensure(inst) = encoding;
+        match self.isa.encode(dfg, &dfg[inst], ty) {
+            Ok(e) => *self.encodings.ensure(inst) = e,
+            Err(_) => panic!("Can't encode {}", dfg.display_inst(inst, self.isa)),
+        }
 
         // Update live ranges.
         self.liveness.create_dead(copy, inst, Affinity::Reg(rci));