Move confusing comment about otherwise blocks in lower_match_tree

This comment was historically inside a block guarded by
`if let Some(otherwise_block) = otherwise`.

When rust-lang#120978 made the otherwise block non-optional, it also flattened that
region of code. Doing so left this comment awkwardly stranded aboven an
unrelated line of code, without its original context.

We can restore that context by moving it above the declaration of `otherwise`.

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -385,15 +385,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         let fake_borrows = match_has_guard
             .then(|| util::FakeBorrowCollector::collect_fake_borrows(self, candidates));
 
+        // See the doc comment on `match_candidates` for why we have an
+        // otherwise block. Match checking will ensure this is actually
+        // unreachable.
         let otherwise_block = self.cfg.start_new_block();
 
         // This will generate code to test scrutinee_place and
         // branch to the appropriate arm block
         self.match_candidates(match_start_span, scrutinee_span, block, otherwise_block, candidates);
 
-        // See the doc comment on `match_candidates` for why we may have an
-        // otherwise block. Match checking will ensure this is actually
-        // unreachable.
         let source_info = self.source_info(scrutinee_span);
 
         // Matching on a `scrutinee_place` with an uninhabited type doesn't