[SROA] Avoid expensive isComplete() call (NFC)

llvm#83381 introduced a call
to PHINode::isComplete() in Mem2Reg, which is O(n^2) in the number
of predecessors, resulting in pathological compile-time regressions
for cases with many predecessors.

Remove the isComplete() check and instead cache the attribute
lookup, to only perform it once per function. Actually setting
the FMF flag is cheap.

llvm/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

@@ -393,6 +393,9 @@ struct PromoteMem2Reg {
   /// Lazily compute the number of predecessors a block has.
   DenseMap<const BasicBlock *, unsigned> BBNumPreds;
 
+  /// Whether the function has the no-signed-zeros-fp-math attribute set.
+  bool NoSignedZeros = false;
+
 public:
   PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
                  AssumptionCache *AC)
@@ -740,6 +743,8 @@ void PromoteMem2Reg::run() {
   LargeBlockInfo LBI;
   ForwardIDFCalculator IDF(DT);
 
+  NoSignedZeros = F.getFnAttribute("no-signed-zeros-fp-math").getValueAsBool();
+
   for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
     AllocaInst *AI = Allocas[AllocaNum];
 
@@ -1128,10 +1133,7 @@ void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
         // on the phi node generated at this stage, fabs folding does not
         // happen. So, we try to infer nsz flag from the function attributes to
         // enable this fabs folding.
-        if (APN->isComplete() && isa<FPMathOperator>(APN) &&
-            BB->getParent()
-                ->getFnAttribute("no-signed-zeros-fp-math")
-                .getValueAsBool())
+        if (isa<FPMathOperator>(APN) && NoSignedZeros)
           APN->setHasNoSignedZeros(true);
 
         // The currently active variable for this block is now the PHI.