Generate a RECIPE_PREDICATES table for each ISA.

It turns out that most encoding predicates are expressed as recipe
predicates. This means that the encoding tables can be more compact
since we can check the recipe predicate separately from individual
instruction predicates, and the recipe number is already present in the
table.

- Don't combine recipe and encoding-specific predicates when creating an
  Encoding. Keep them separate.
- Generate a table of recipe predicates with function pointers. Many of
  these are null.
- Check any recipe predicate before accepting a recipe+bits pair.

This has the effect of making almost all instruction predicates
CODE_ALWAYS.

lib/cretonne/src/isa/intel/mod.rs

@@ -73,6 +73,7 @@ impl TargetIsa for Isa {
                 .and_then(|enclist_offset| {
                               Ok(Encodings::new(enclist_offset,
                                                 &enc_tables::ENCLISTS[..],
+                                                &enc_tables::RECIPE_PREDICATES[..],
                                                 inst,
                                                 enc_tables::check_instp,
                                                 self.isa_flags.predicate_view()))