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/interpret/machine.rs

@@ -243,10 +243,10 @@ pub trait Machine<'mir, 'tcx>: Sized {
         ecx.stack_mut()[frame].locals[local].access_mut()
     }
 
-    /// Called before a basic block terminator is executed.
-    /// You can use this to detect endlessly running programs.
+    /// Called when the interpreter encounters a `StatementKind::ConstEvalCounter` instruction.
+    /// You can use this to detect long or endlessly running programs.
     #[inline]
-    fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
+    fn increment_const_eval_counter(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
         Ok(())
     }