Split the Encoding data type into two u16 values.

This hardcodes the division line between the recipe bits and the
encoding bits. It does not seem that any ISA will need more than 16 bits for
either.

cranelift/src/libcretonne/isa/mod.rs

@@ -100,18 +100,11 @@ pub trait TargetIsa {
 /// encoding *bits*. The recipe determines the native instruction format and the mapping of
 /// operands to encoded bits. The encoding bits provide additional information to the recipe,
 /// typically parts of the opcode.
-pub struct Encoding(u32);
+pub struct Encoding(u16, u16);
 
 impl Encoding {
-    /// Create a new `Encoding` containing `(recipe, bits)`. The `num_bits` parameter is the
-    /// ISA-dependent size of `bits`.
-    pub fn new(recipe: u32, bits: u32, num_bits: u8) -> Encoding {
-        Encoding((recipe << num_bits) | bits)
-    }
-
-    /// Split the encoding into two parts: `(recipe, bits)`. Only the target ISA knows how many
-    /// bits are in each part.
-    pub fn split(&self, num_bits: u8) -> (u32, u32) {
-        (self.0 >> num_bits, self.0 & ((1 << num_bits) - 1))
+    /// Create a new `Encoding` containing `(recipe, bits)`.
+    pub fn new(recipe: u16, bits: u16) -> Encoding {
+        Encoding(recipe, bits)
     }
 }