Remove in_band_lifetimes from rustc_mir_transform

This one is a heavy `'tcx` user.

Two interesting ones:

This one had the `'tcx` declared on the function, despite the trait taking a `'tcx`:
```diff
-impl Visitor<'_> for UsedLocals {
+impl<'tcx> Visitor<'tcx> for UsedLocals {
     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
```

This one use in-band for one, and underscore for the other:
```diff
-pub fn remove_dead_blocks(tcx: TyCtxt<'tcx>, body: &mut Body<'_>) {
+pub fn remove_dead_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
```

compiler/rustc_mir_transform/src/instcombine.rs

@@ -38,7 +38,7 @@ struct InstCombineContext<'tcx, 'a> {
     local_decls: &'a LocalDecls<'tcx>,
 }
 
-impl<'tcx, 'a> InstCombineContext<'tcx, 'a> {
+impl<'tcx> InstCombineContext<'tcx, '_> {
     fn should_combine(&self, source_info: &SourceInfo, rvalue: &Rvalue<'tcx>) -> bool {
         self.tcx.consider_optimizing(|| {
             format!("InstCombine - Rvalue: {:?} SourceInfo: {:?}", rvalue, source_info)