Add back Machine::before_terminator(...) method

Added it back because it's used by Miri, but in the compiler itself, it
will not do anything (just return `Ok(())`.

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -561,6 +561,11 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         throw_unsup_format!("pointer arithmetic or comparison is not supported at compile-time");
     }
 
+    // Not used here, but used by Miri (see `src/tools/miri/src/machine.rs`).
+    fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
+        Ok(())
+    }
+
     fn increment_const_eval_counter(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
         // The step limit has already been hit in a previous call to `increment_const_eval_counter`.
         if ecx.machine.steps_remaining == 0 {

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -243,6 +243,12 @@ pub trait Machine<'mir, 'tcx>: Sized {
         ecx.stack_mut()[frame].locals[local].access_mut()
     }
 
+    /// Called before a basic block terminator is executed.
+    #[inline]
+    fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
+        Ok(())
+    }
+
     /// Called when the interpreter encounters a `StatementKind::ConstEvalCounter` instruction.
     /// You can use this to detect long or endlessly running programs.
     #[inline]

compiler/rustc_const_eval/src/interpret/step.rs

@@ -62,6 +62,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             return Ok(true);
         }
 
+        M::before_terminator(self)?;
+
         let terminator = basic_block.terminator();
         self.terminator(terminator)?;
         Ok(true)