Support for debug-labels. (#27)

Support for debug-labels.

If the client adds labels to vregs across ranges of instructions in the
input program, the regalloc will provide metadata in the `Output` that
describes the `Allocation`s in which each such vreg is stored for those
ranges. This allows the client to emit debug metadata telling a debugger
where to find program values at each point in the program.

src/lib.rs

@@ -958,7 +958,14 @@ pub trait Function {
     /// match -- specifically, the returned metadata may cover only a
     /// subset of the requested ranges -- if the value is not live for
     /// the entire requested ranges.
-    fn debug_value_labels(&self) -> &[(Inst, Inst, VReg, u32)] {
+    ///
+    /// The instruction indices imply a program point just *before*
+    /// the instruction.
+    ///
+    /// Preconditions: we require this slice to be sorted in
+    /// lexicographic order (i.e., first by vreg, then by instruction
+    /// index), and we require the ranges to be non-overlapping.
+    fn debug_value_labels(&self) -> &[(VReg, Inst, Inst, u32)] {
         &[]
     }