Rollup merge of #126077 - oli-obk:revert_is_mir_available, r=BoxyUwU

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

This reverts commit e5cba17b84.

turns out SMIR still needs it (https://github.com/model-checking/kani/issues/3218). I'll create a full plan and MCP for what I intended this to be a part of. Maybe my plan is nonsense anyway.

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