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/mod.rs

@@ -28,4 +28,4 @@ pub use ir::dfg::{DataFlowGraph, ValueDef};
 pub use ir::layout::{Layout, Cursor};
 pub use ir::function::Function;
 pub use ir::builder::InstBuilder;
-pub use ir::progpoint::{ProgramPoint, ProgramOrder};
+pub use ir::progpoint::{ProgramPoint, ProgramOrder, ExpandedProgramPoint};