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_middle/src/mir/syntax.rs

@@ -286,7 +286,10 @@ pub enum StatementKind<'tcx> {
     /// This is permitted for both generators and ADTs. This does not necessarily write to the
     /// entire place; instead, it writes to the minimum set of bytes as required by the layout for
     /// the type.
-    SetDiscriminant { place: Box<Place<'tcx>>, variant_index: VariantIdx },
+    SetDiscriminant {
+        place: Box<Place<'tcx>>,
+        variant_index: VariantIdx,
+    },
 
     /// Deinitializes the place.
     ///
@@ -355,6 +358,8 @@ pub enum StatementKind<'tcx> {
     /// This avoids adding a new block and a terminator for simple intrinsics.
     Intrinsic(Box<NonDivergingIntrinsic<'tcx>>),
 
+    ConstEvalCounter,
+
     /// No-op. Useful for deleting instructions without affecting statement indices.
     Nop,
 }