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/const_eval/eval_queries.rs

@@ -22,6 +22,8 @@ use crate::interpret::{
     RefTracking, StackPopCleanup,
 };
 
+use tracing::info;
+
 const NOTE_ON_UNDEFINED_BEHAVIOR_ERROR: &str = "The rules on what exactly is undefined behavior aren't clear, \
      so this check might be overzealous. Please open an issue on the rustc \
      repository if you believe it should not be considered undefined behavior.";
@@ -33,6 +35,7 @@ fn eval_body_using_ecx<'mir, 'tcx>(
     body: &'mir mir::Body<'tcx>,
 ) -> InterpResult<'tcx, MPlaceTy<'tcx>> {
     debug!("eval_body_using_ecx: {:?}, {:?}", cid, ecx.param_env);
+    info!("HERE body is {:#?}", body);
     let tcx = *ecx.tcx;
     assert!(
         cid.promoted.is_some()

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -369,6 +369,7 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         }
     }
 
+    #[instrument(skip(ecx), ret)]
     fn load_mir(
         ecx: &InterpCx<'mir, 'tcx, Self>,
         instance: ty::InstanceDef<'tcx>,