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/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]