Use checked NonZero constructor in MIR move path indices

… to protect against UB in the unlikely case that `idx + 1` overflows.

Author: SimonSapin

src/librustc_mir/dataflow/move_paths/mod.rs

@@ -42,7 +42,7 @@ pub(crate) mod indexes {
 
             impl Idx for $Index {
                 fn new(idx: usize) -> Self {
-                    unsafe { $Index(NonZero::new_unchecked(idx + 1)) }
+                    $Index(NonZero::new(idx + 1).unwrap())
                 }
                 fn index(self) -> usize {
                     self.0.get() - 1