Add a ValueLoc type and the locations table.

This table holds the result of register allocation.

lib/cretonne/src/ir/valueloc.rs

@@ -0,0 +1,24 @@
+//! Value locations.
+//!
+//! The register allocator assigns every SSA value to either a register or a stack slot. This
+//! assignment is represented by a `ValueLoc` object.
+
+use isa::RegUnit;
+use ir::StackSlot;
+
+/// Value location.
+#[derive(Copy, Clone, Debug)]
+pub enum ValueLoc {
+    /// This value has not been assigned to a location yet.
+    Unassigned,
+    /// Value is assigned to a register.
+    Reg(RegUnit),
+    /// Value is assigned to a stack slot.
+    Stack(StackSlot),
+}
+
+impl Default for ValueLoc {
+    fn default() -> Self {
+        ValueLoc::Unassigned
+    }
+}