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/encoding.rs

@@ -1,6 +1,7 @@
 //! The `Encoding` struct.
 
 use std::fmt;
+use isa::constraints::RecipeConstraints;
 
 /// Bits needed to encode an instruction as binary machine code.
 ///
@@ -76,3 +77,28 @@ impl fmt::Display for DisplayEncoding {
         }
     }
 }
+
+/// Information about all the encodings in this ISA.
+#[derive(Clone)]
+pub struct EncInfo {
+    /// Constraints on value operands per recipe.
+    pub constraints: &'static [RecipeConstraints],
+
+    /// Names of encoding recipes.
+    pub names: &'static [&'static str],
+}
+
+impl EncInfo {
+    /// Get the value operand constraints for `enc` if it is a legal encoding.
+    pub fn operand_constraints(&self, enc: Encoding) -> Option<&RecipeConstraints> {
+        self.constraints.get(enc.recipe())
+    }
+
+    /// Create an object that can display an ISA-dependent encoding properly.
+    pub fn display(&self, enc: Encoding) -> DisplayEncoding {
+        DisplayEncoding {
+            encoding: enc,
+            recipe_names: self.names,
+        }
+    }
+}