Remove a FIXME and explain the decision

Author: oli-obk

compiler/rustc_mir/src/util/pretty.rs

@@ -274,7 +274,8 @@ pub fn write_mir_pretty<'tcx>(
     let mut first = true;
     for def_id in dump_mir_def_ids(tcx, single) {
         let body = match tcx.hir().body_const_context(def_id.expect_local()) {
-            // FIXME: print both MIRs for `const fn`?
+            // For `const fn` we want to render the optimized MIR. If you want the mir used in
+            // ctfe, you can dump the MIR after the `Deaggregator` optimization pass.
             None | Some(rustc_hir::ConstContext::ConstFn) => tcx.optimized_mir(def_id),
             Some(_) => tcx.mir_for_ctfe(def_id),
         };