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

[Graham Hunter via llvm-commits]

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@@ -99,6 +99,26 @@ class MapVector {
     return try_emplace_impl(Key).first->second;
   }
 
+  [[nodiscard]] inline auto keys() {
+    return map_range(
+        Vector, [](const std::pair<KeyT, ValueT> &KV) { return KV.first; });
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huntergr-arm wrote:

Perhaps `make_first_range` and `make_second_range` would be better than `map_range` for these functions?

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