Replace terminator-based const eval limit

- Remove logic that limits const eval based on terminators, and use the
  stable metric instead (back edges + fn calls)
- Add unstable flag `tiny-const-eval-limit` to add UI tests that do not
  have to go up to the regular 2M step limit

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -561,8 +561,8 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         throw_unsup_format!("pointer arithmetic or comparison is not supported at compile-time");
     }
 
-    fn before_terminator(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
-        // The step limit has already been hit in a previous call to `before_terminator`.
+    fn increment_const_eval_counter(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
+        // The step limit has already been hit in a previous call to `increment_const_eval_counter`.
         if ecx.machine.steps_remaining == 0 {
             return Ok(());
         }