[LV][AArch64] Resolve test failure due use of unordered container

[sushgokh]

Summary

D143422 test AArch64/reg-usage.ll has an issue with the output ordering due to use of unordered container. This was discovered by
-DLLVM_REVERSE_ITERATION:BOOL=ON cmake option.

This patch tries to address it by making use of ordered container.

[fhahn]

LGTM, thanks! The commit title could be a bit more descriptive by explicitly mentioning what the code change is, e.g. something like: [LV] Use SetVector to collect invariants in calculateRegisterUsage.

Also, this is not AArch64 specific, so it would be good to drop the [AArch64] part.

[sushgokh]

In D145472#4174810, @fhahn wrote:

LGTM, thanks! The commit title could be a bit more descriptive by explicitly mentioning what the code change is, e.g. something like: [LV] Use SetVector to collect invariants in calculateRegisterUsage.

Also, this is not AArch64 specific, so it would be good to drop the [AArch64] part.

Thanks @fhahn for the suggestion.

Regarding inclusion of AArch64 in title, I thought this was more about AArch64 test failure. But I was incorrect. Will keep this in mind that its more about the change

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

LoopVectorizationCostModel::calculateRegisterUsage(ArrayRef<ElementCount> VFs) {
   SmallVector<Instruction *, 64> IdxToInstr;
   // Marks the end of each interval.
   IntervalMap EndPoint;
   // Saves the list of instruction indices that are used in the loop.
   SmallPtrSet<Instruction *, 8> Ends;
   // Saves the list of values that are used in the loop but are defined outside
   // the loop (not including non-instruction values such as arguments and
   // constants).
-  SmallPtrSet<Instruction *, 8> LoopInvariants;
+  SmallSetVector<Instruction *, 8> LoopInvariants;
 
   for (BasicBlock *BB : make_range(DFS.beginRPO(), DFS.endRPO())) {
     for (Instruction &I : BB->instructionsWithoutDebug()) {
       IdxToInstr.push_back(&I);
 
       // Save the end location of each USE.
       for (Value *U : I.operands()) {
         auto *Instr = dyn_cast<Instruction>(U);