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_session/src/options.rs

@@ -1616,6 +1616,8 @@ options! {
         "measure time of each LLVM pass (default: no)"),
     time_passes: bool = (false, parse_bool, [UNTRACKED],
         "measure time of each rustc pass (default: no)"),
+    tiny_const_eval_limit: bool = (false, parse_bool, [TRACKED],
+        "sets a tiny, non-configurable limit for const eval; useful for compiler tests"),
     #[rustc_lint_opt_deny_field_access("use `Session::tls_model` instead of this field")]
     tls_model: Option<TlsModel> = (None, parse_tls_model, [TRACKED],
         "choose the TLS model to use (`rustc --print tls-models` for details)"),