Add a LiveRange data structure.

We will track live ranges separately for each SSA value, rather than per
virtual register like LLVM does.

This is the basis for a register allocator, so place it in a new
regalloc module.

lib/cretonne/src/ir/progpoint.rs

@@ -36,9 +36,9 @@ impl From<Ebb> for ProgramPoint {
 /// represent.
 #[derive(PartialEq, Eq, Clone, Copy)]
 pub enum ExpandedProgramPoint {
-    // An instruction in the function.
+    /// An instruction in the function.
     Inst(Inst),
-    // An EBB header.
+    /// An EBB header.
     Ebb(Ebb),
 }