Revert "Use the HIR instead of mir_keys for determining whether something will have a MIR body."

This reverts commit e5cba17b84bf7bf755686e8bb36aa3775ef53f77.

compiler/rustc_mir_transform/src/lib.rs

@@ -211,7 +211,7 @@ fn remap_mir_for_const_eval_select<'tcx>(
 }
 
 fn is_mir_available(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
-    tcx.hir().maybe_body_owned_by(def_id).is_some()
+    tcx.mir_keys(()).contains(&def_id)
 }
 
 /// Finds the full set of `DefId`s within the current crate that have