Move encoding-related information into an EncInfo struct.

The tables returned by recipe_names() and recipe_constraints() are now
collected into an EncInfo struct that is available from
TargetIsa::encoding_info(). This is equivalent to the register bank
tables available fro TargetIsa::register_info().

This cleans of the TargetIsa interface and makes it easier to add
encoding-related information.

lib/cretonne/src/isa/riscv/enc_tables.rs

@@ -1,11 +1,12 @@
 //! Encoding tables for RISC-V.
 
 use ir::condcodes::IntCC;
-use ir::{Opcode, InstructionData};
 use ir::types;
-use predicates;
-use isa::enc_tables::{Level1Entry, Level2Entry};
+use ir::{Opcode, InstructionData};
+use isa::EncInfo;
 use isa::constraints::*;
+use isa::enc_tables::{Level1Entry, Level2Entry};
+use predicates;
 use super::registers::*;
 
 // Include the generated encoding tables:
@@ -13,4 +14,5 @@ use super::registers::*;
 // - `LEVEL1_RV64`
 // - `LEVEL2`
 // - `ENCLIST`
+// - `INFO`
 include!(concat!(env!("OUT_DIR"), "/encoding-riscv.rs"));