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_mir_dataflow/src/impls/liveness.rs

@@ -271,6 +271,7 @@ impl<'a, 'tcx> Analysis<'tcx> for MaybeTransitiveLiveLocals<'a> {
             | StatementKind::AscribeUserType(..)
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
+            | StatementKind::ConstEvalCounter
             | StatementKind::Nop => None,
         };
         if let Some(destination) = destination {

compiler/rustc_mir_dataflow/src/impls/storage_liveness.rs

@@ -141,6 +141,7 @@ impl<'mir, 'tcx> crate::GenKillAnalysis<'tcx> for MaybeRequiresStorage<'mir, 'tc
             StatementKind::AscribeUserType(..)
             | StatementKind::Coverage(..)
             | StatementKind::FakeRead(..)
+            | StatementKind::ConstEvalCounter
             | StatementKind::Nop
             | StatementKind::Retag(..)
             | StatementKind::Intrinsic(..)

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -331,6 +331,7 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
             | StatementKind::AscribeUserType(..)
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
+            | StatementKind::ConstEvalCounter
             | StatementKind::Nop => {}
         }
     }

compiler/rustc_mir_dataflow/src/value_analysis.rs

@@ -84,7 +84,8 @@ pub trait ValueAnalysis<'tcx> {
             StatementKind::Retag(..) => {
                 // We don't track references.
             }
-            StatementKind::Nop
+            StatementKind::ConstEvalCounter
+            | StatementKind::Nop
             | StatementKind::FakeRead(..)
             | StatementKind::Coverage(..)
             | StatementKind::AscribeUserType(..) => (),