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/enc_tables.rs

@@ -1,7 +1,7 @@
 //! Encoding tables for Intel ISAs.
 
 use ir::{self, types, Opcode};
-use isa::EncInfo;
+use isa;
 use isa::constraints::*;
 use isa::enc_tables::*;
 use isa::encoding::RecipeSizing;
@@ -9,3 +9,4 @@ use predicates;
 use super::registers::*;
 
 include!(concat!(env!("OUT_DIR"), "/encoding-intel.rs"));
+include!(concat!(env!("OUT_DIR"), "/legalize-intel.rs"));