Rollup merge of #98930 - tmiasko:pub-basic-blocks, r=oli-obk

Make MIR basic blocks field public

This makes it possible to mutably borrow different fields of the MIR
body without resorting to methods like `basic_blocks_local_decls_mut_and_var_debug_info`.

To preserve validity of control flow graph caches in the presence of
modifications, a new struct `BasicBlocks` wraps together basic blocks
and control flow graph caches.

The `BasicBlocks` dereferences to `IndexVec<BasicBlock, BasicBlockData>`.
On the other hand a mutable access requires explicit `as_mut()` call.

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -15,7 +15,7 @@ pub fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     fx: &FunctionCx<'a, 'tcx, Bx>,
 ) -> BitSet<mir::Local> {
     let mir = fx.mir;
-    let dominators = mir.dominators();
+    let dominators = mir.basic_blocks.dominators();
     let locals = mir
         .local_decls
         .iter()