[Scalar] Use SmallSetVector instead of SmallVector (NFC) (#154678)

insertParsePoints collects live variables and then deduplicate them
while retaining the original insertion order, which is exactly what
SetVector is designed for.  This patch replaces SmallVector with
SetSmallVector while deleting unique_unsorted.

While we are at it, this patch reduces the number of inline elements
to a reasonable level for linear search.

Author: kazutakahirata

llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

@@ -2214,14 +2214,6 @@ static void relocationViaAlloca(
 #endif
 }
 
-/// Implement a unique function which doesn't require we sort the input
-/// vector.  Doing so has the effect of changing the output of a couple of
-/// tests in ways which make them less useful in testing fused safepoints.
-template <typename T> static void unique_unsorted(SmallVectorImpl<T> &Vec) {
-  SmallPtrSet<T, 8> Seen;
-  erase_if(Vec, [&](const T &V) { return !Seen.insert(V).second; });
-}
-
 /// Insert holders so that each Value is obviously live through the entire
 /// lifetime of the call.
 static void insertUseHolderAfter(CallBase *Call, const ArrayRef<Value *> Values,
@@ -2839,15 +2831,17 @@ static bool insertParsePoints(Function &F, DominatorTree &DT,
   }
   PointerToBase.clear();
 
-  // Do all the fixups of the original live variables to their relocated selves
-  SmallVector<Value *, 128> Live;
+  // Do all the fixups of the original live variables to their relocated selves.
+  // A SmallSetVector is used to collect live variables while retaining the
+  // order in which we add them, which is important for reproducible tests.
+  SmallSetVector<Value *, 16> Live;
   for (const PartiallyConstructedSafepointRecord &Info : Records) {
     // We can't simply save the live set from the original insertion.  One of
     // the live values might be the result of a call which needs a safepoint.
     // That Value* no longer exists and we need to use the new gc_result.
     // Thankfully, the live set is embedded in the statepoint (and updated), so
     // we just grab that.
-    llvm::append_range(Live, Info.StatepointToken->gc_live());
+    Live.insert_range(Info.StatepointToken->gc_live());
 #ifndef NDEBUG
     // Do some basic validation checking on our liveness results before
     // performing relocation.  Relocation can and will turn mistakes in liveness
@@ -2867,7 +2861,6 @@ static bool insertParsePoints(Function &F, DominatorTree &DT,
     }
 #endif
   }
-  unique_unsorted(Live);
 
 #ifndef NDEBUG
   // Validation check
@@ -2876,7 +2869,7 @@ static bool insertParsePoints(Function &F, DominatorTree &DT,
            "must be a gc pointer type");
 #endif
 
-  relocationViaAlloca(F, DT, Live, Records);
+  relocationViaAlloca(F, DT, Live.getArrayRef(), Records);
   return !Records.empty();
 }