Use IndexVec::indices instead of reimplementing it

The change is limited to the iteration over indices instead of using
`basic_blocks_mut()` directly, in the case the previous implementation
intentionally avoided invalidating the caches stored in MIR body.

compiler/rustc_mir_transform/src/uninhabited_enum_branching.rs

@@ -3,8 +3,7 @@
 use crate::MirPass;
 use rustc_data_structures::stable_set::FxHashSet;
 use rustc_middle::mir::{
-    BasicBlock, BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets,
-    TerminatorKind,
+    BasicBlockData, Body, Local, Operand, Rvalue, StatementKind, SwitchTargets, TerminatorKind,
 };
 use rustc_middle::ty::layout::TyAndLayout;
 use rustc_middle::ty::{Ty, TyCtxt};
@@ -73,10 +72,7 @@ impl<'tcx> MirPass<'tcx> for UninhabitedEnumBranching {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
         trace!("UninhabitedEnumBranching starting for {:?}", body.source);
 
-        let basic_block_count = body.basic_blocks().len();
-
-        for bb in 0..basic_block_count {
-            let bb = BasicBlock::from_usize(bb);
+        for bb in body.basic_blocks().indices() {
             trace!("processing block {:?}", bb);
 
             let Some(discriminant_ty) = get_switched_on_type(&body.basic_blocks()[bb], tcx, body) else {