Make register copies for incompatible operands.

An instruction may have fixed operand constraints that make it
impossibly to use a single register value to satisfy two at a time.

Detect when the same value is used for multiple fixed register operands
and insert copies during the spilling pass.

lib/cretonne/src/ir/entities.rs

@@ -74,7 +74,7 @@ impl Ebb {
 }
 
 /// An opaque reference to an SSA value.
-#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash, Debug, PartialOrd, Ord)]
 pub struct Value(u32);
 entity_impl!(Value, "v");