[RS4GC] Don't copy ADT's unneccessarily, NFCI

[sanjoy]

Summary

Use const auto & instead of auto in makeStatepointExplicit.

[reames]

LGTM. This didn't really need review.

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

 // Replace an existing gc.statepoint with a new one and a set of gc.relocates
 // which make the relocations happening at this safepoint explicit.
 //
 // WARNING: Does not do any fixup to adjust users of the original live
 // values.  That's the callers responsibility.
 static void
 makeStatepointExplicit(DominatorTree &DT, const CallSite &CS,
                        PartiallyConstructedSafepointRecord &Result) {
-  auto LiveSet = Result.LiveSet;
-  auto PointerToBase = Result.PointerToBase;
+  const auto &LiveSet = Result.LiveSet;
+  const auto &PointerToBase = Result.PointerToBase;
 
   // Convert to vector for efficient cross referencing.
   SmallVector<Value *, 64> BaseVec, LiveVec;
   LiveVec.reserve(LiveSet.size());
   BaseVec.reserve(LiveSet.size());
   for (Value *L : LiveSet) {
     LiveVec.push_back(L);
     assert(PointerToBase.count(L));
-    Value *Base = PointerToBase[L];
+    Value *Base = PointerToBase.find(L)->second;
     BaseVec.push_back(Base);
   }
   assert(LiveVec.size() == BaseVec.size());
 
   // To make the output IR slightly more stable (for use in diffs), ensure a
   // fixed order of the values in the safepoint (by sorting the value name).
   // The order is otherwise meaningless.
   StabilizeOrder(BaseVec, LiveVec);