Add a verifier

The current implementation only performs a few basic checks.

cranelift/src/libcretonne/ir/layout.rs

@@ -55,7 +55,7 @@ impl Layout {
 /// can only be removed from the layout when it is empty.
 ///
 /// Since every EBB must end with a terminator instruction which cannot fall through, the layout of
-/// EBBs does not affect the semantics of the program.
+/// EBBs do not affect the semantics of the program.
 ///
 impl Layout {
     /// Is `ebb` currently part of the layout?
@@ -188,6 +188,11 @@ impl Layout {
         ebb_node.last_inst = inst;
     }
 
+    /// Fetch an ebb's last instruction.
+    pub fn last_inst(&self, ebb: Ebb) -> Inst {
+        self.ebbs[ebb].last_inst
+    }
+
     /// Insert `inst` before the instruction `before` in the same EBB.
     pub fn insert_inst(&mut self, inst: Inst, before: Inst) {
         assert_eq!(self.inst_ebb(inst), None);