Clarify why InhabitedPredicate::instantiate_opt exists

At first glance, the extra casework seems pointless and needlessly
error-prone. Clarify that there is a reason for it being there.

Author: meithecatte

Diff for: compiler/rustc_middle/src/ty/inhabitedness/inhabited_predicate.rs

@@ -236,6 +236,11 @@ impl<'tcx> InhabitedPredicate<'tcx> {
         self.instantiate_opt(tcx, args).unwrap_or(self)
     }
 
+    /// Same as [`Self::instantiate`], but if there is no generics to
+    /// instantiate, returns `None`. This is useful because it lets us avoid
+    /// allocating a recursive copy of everything when the result is unchanged.
+    ///
+    /// Only used to implement `instantiate` itself.
     fn instantiate_opt(self, tcx: TyCtxt<'tcx>, args: ty::GenericArgsRef<'tcx>) -> Option<Self> {
         match self {
             Self::ConstIsZero(c) => {