[LoopVectorize] Use SetVector to track uniform uses to prevent non-determinism.

[huihuiz]

Summary

Use SetVector instead of SmallPtrSet to track values with uniform use. Doing this
can help avoid non-determinism caused by iterating over unordered containers.

This bug was found with reverse iteration turning on,
--extra-llvm-cmake-variables="-DLLVM_REVERSE_ITERATION=ON".
Failing LLVM test consecutive-ptr-uniforms.ll .

[huihuiz]

Update diff to use SetVector.

[MaskRay]

Looks great!

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

auto isVectorizedMemAccessUse = [&](Instruction *I, Value *Ptr) -> bool {
     return getLoadStorePointerOperand(I) == Ptr && isUniformDecision(I, VF);
   };
 
   // Holds a list of values which are known to have at least one uniform use.
   // Note that there may be other uses which aren't uniform.  A "uniform use"
   // here is something which only demands lane 0 of the unrolled iterations;
   // it does not imply that all lanes produce the same value (e.g. this is not
   // the usual meaning of uniform)
-  SmallPtrSet<Value *, 8> HasUniformUse;
+  SetVector<Value *> HasUniformUse;
 
   // Scan the loop for instructions which are either a) known to have only
   // lane 0 demanded or b) are uses which demand only lane 0 of their operand.
   for (auto *BB : TheLoop->blocks())
     for (auto &I : *BB) {
       // If there's no pointer operand, there's nothing to do.
       auto *Ptr = getLoadStorePointerOperand(&I);
       if (!Ptr)