Move variable into the only branch where it is relevant

At the `if` branch `filter` (the `let` binding) is `None` iff `filter` (the parameter) was `None`.
We can branch on the parameter, move the binding into the `if`, and the complexity of handling
`Option<Option<_>` largely dissolves.

compiler/rustc_metadata/src/rmeta/decoder.rs

@@ -1345,15 +1345,14 @@ fn get_implementations_for_trait(
             return &[];
         }
 
-        // Do a reverse lookup beforehand to avoid touching the crate_num
-        // hash map in the loop below.
-        let filter = match filter.map(|def_id| self.reverse_translate_def_id(def_id)) {
-            Some(Some(def_id)) => Some((def_id.krate.as_u32(), def_id.index)),
-            Some(None) => return &[],
-            None => None,
-        };
+        if let Some(def_id) = filter {
+            // Do a reverse lookup beforehand to avoid touching the crate_num
+            // hash map in the loop below.
+            let filter = match self.reverse_translate_def_id(def_id) {
+                Some(def_id) => (def_id.krate.as_u32(), def_id.index),
+                None => return &[],
+            };
 
-        if let Some(filter) = filter {
             if let Some(impls) = self.trait_impls.get(&filter) {
                 tcx.arena.alloc_from_iter(
                     impls.decode(self).map(|(idx, simplified_self_ty)| {