5474f56bcf
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.