Auto merge of #30757 - michaelwoerister:mir-visitor-cleanup, r=jroesch

After a call to `visit_def_id()` missing in `mir::visit::Visitor` but not `mir::visit::MutVisitor` has caused me a couple hours of error hunting, I decided I'd take the time to get rid of the code duplication between the two implementations.

cc @rust-lang/compiler

src/librustc/mir/repr.rs

@@ -21,6 +21,7 @@ use std::ascii;
 use std::borrow::{Cow, IntoCow};
 use std::fmt::{self, Debug, Formatter, Write};
 use std::{iter, u32};
+use std::ops::{Index, IndexMut};
 
 /// Lowered representation of a single function.
 #[derive(RustcEncodable, RustcDecodable)]
@@ -68,6 +69,22 @@ impl<'tcx> Mir<'tcx> {
     }
 }
 
+impl<'tcx> Index<BasicBlock> for Mir<'tcx> {
+    type Output = BasicBlockData<'tcx>;
+
+    #[inline]
+    fn index(&self, index: BasicBlock) -> &BasicBlockData<'tcx> {
+        self.basic_block_data(index)
+    }
+}
+
+impl<'tcx> IndexMut<BasicBlock> for Mir<'tcx> {
+    #[inline]
+    fn index_mut(&mut self, index: BasicBlock) -> &mut BasicBlockData<'tcx> {
+        self.basic_block_data_mut(index)
+    }
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // Mutability and borrow kinds