[RegAllocGreedy] Use simpler map class for EvicteeInfo. NFCI.

RegAlloc keeps a insertion-time ordered map of evictee information,
but we only use membership. Replace MapVector with contextually
equivalent DenseMap which is smaller and faster.

llvm-svn: 333981
diff --git a/llvm/lib/CodeGen/RegAllocGreedy.cpp b/llvm/lib/CodeGen/RegAllocGreedy.cpp
index 182d966..d9ff76b 100644
--- a/llvm/lib/CodeGen/RegAllocGreedy.cpp
+++ b/llvm/lib/CodeGen/RegAllocGreedy.cpp
@@ -292,7 +292,7 @@
   public:
     using EvictorInfo =
         std::pair<unsigned /* evictor */, unsigned /* physreg */>;
-    using EvicteeInfo = llvm::MapVector<unsigned /* evictee */, EvictorInfo>;
+    using EvicteeInfo = llvm::DenseMap<unsigned /* evictee */, EvictorInfo>;
 
   private:
     /// Each Vreg that has been evicted in the last stage of selectOrSplit will