Create stable metric to measure long computation in Const Eval

This patch adds a `MirPass` that tracks the number of back-edges and
function calls in the CFG, adds a new MIR instruction to increment a
counter every time they are encountered during Const Eval, and emit a
warning if a configured limit is breached.

compiler/rustc_const_eval/src/interpret/step.rs

@@ -129,6 +129,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             // FIXME(#73156): Handle source code coverage in const eval
             Coverage(..) => {}
 
+            // FIXME(bryangarza): Update this to do some logic!!!
+            ConstEvalCounter => {
+                self.increment_const_eval_counter();
+            }
+
             // Defined to do nothing. These are added by optimization passes, to avoid changing the
             // size of MIR constantly.
             Nop => {}