Add regspill and regfill instructions.

These are parallels to the existing regmove instruction, but the divert
the value to and from a stack slot.

Like regmove diversions, this is a temporary diversion that must be
local to the EBB.

lib/cretonne/src/verifier/mod.rs

@@ -317,6 +317,12 @@ impl<'a> Verifier<'a> {
             HeapAddr { heap, .. } => {
                 self.verify_heap(inst, heap)?;
             }
+            RegSpill { dst, .. } => {
+                self.verify_stack_slot(inst, dst)?;
+            }
+            RegFill { src, .. } => {
+                self.verify_stack_slot(inst, src)?;
+            }
 
             // Exhaustive list so we can't forget to add new formats
             Unary { .. } |