coverage: Make yank_to_spantree_root iterative instead of recursive

This avoids having to worry about stack space when traversing very long
spantree paths, e.g. when instrumenting a long sequence of if/else statements.

compiler/rustc_mir_transform/src/lib.rs

@@ -1,4 +1,5 @@
 // tidy-alphabetical-start
+#![feature(array_windows)]
 #![feature(assert_matches)]
 #![feature(box_patterns)]
 #![feature(const_type_name)]