WIP: Handle moves between realregs (pregs) and vregs somewhat specially, by converting into operand constraints

Still has a fuzzbug in interaction between R->R and V->R moves. Will
likely rework to make pinned-vreg handling more general but want to save
a checkpoint here; idea for rework:
- set allocs immediately if an Operand is a pinned vreg;
- reserve preg ranges;
- then, in rest of liveness computation / LR construction, convert
  pinned-vregs to operands with constraints, but otherwise do not
  special-case as we do in this commit.

src/checker.rs

@@ -589,6 +589,12 @@ impl<'a, F: Function> Checker<'a, F> {
                         .unwrap()
                         .push(CheckerInst::Move { into: to, from });
                 }
+                &Edit::DefAlloc { .. } => {
+                    unimplemented!(concat!(
+                        "DefAlloc is used only when dealing with pinned vregs, ",
+                        "which are only used by regalloc.rs shim; use checker at that level!"
+                    ));
+                }
                 &Edit::BlockParams {
                     ref vregs,
                     ref allocs,