[llvm] [MapVector] Introduce {keys, values} iterator, use in VPlan (PR #169675)

[Florian Hahn via llvm-commits]

================
@@ -99,6 +99,11 @@ class MapVector {
     return try_emplace_impl(Key).first->second;
   }
 
+  [[nodiscard]] inline auto keys() { return make_first_range(Vector); }
+  [[nodiscard]] inline auto values() { return make_second_range(Vector); }
+  [[nodiscard]] inline auto keys() const { return make_first_range(Vector); }
+  [[nodiscard]] inline auto values() const { return make_second_range(Vector); }
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fhahn wrote:

```suggestion
  [[nodiscard]] inline auto keys() { return make_first_range(Vector); }
  [[nodiscard]] inline auto keys() const { return make_first_range(Vector); }
  [[nodiscard]] inline auto values() { return make_second_range(Vector); }
  [[nodiscard]] inline auto values() const { return make_second_range(Vector); }
```

Keep values(), keys() together?

https://github.com/llvm/llvm-project/pull/169675