[llvm-stress] Fix dead code preventing us generating per-element vector selects

[RKSimon]

Summary

This has been reported several times by the PVS Studio team as well as coming up in some static analysis.

getRandom() % 1 always returns 0 so we never actually test this codepath, (git blame suggests this has always been like this) - given that we have plenty of other "getRandom() & 1" the typo is pretty obvious, and matches the intention in the comment above - with this change we generate a nice mixture of scalar/vector condition selects of vectors.

I don't know llvm-stress that well - but I don't think we guarantee that the same seed value will always generate the same IR for later versions of the program - just that the same binary would.

[lebedev.ri]

Never seen this program before, but the bug is obvious and i don't really believe we'd need to maintain backwards compatibility for previously-buggy inputs.

llvm/tools/llvm-stress/llvm-stress.cpp

void Act() override {
     // Try a bunch of different select configuration until a valid one is found.
     Value *Val0 = getRandomVal();
     Value *Val1 = getRandomValue(Val0->getType());
 
     Type *CondTy = Type::getInt1Ty(Context);
 
     // If the value type is a vector, and we allow vector select, then in 50%
     // of the cases generate a vector select.
-    if (isa<FixedVectorType>(Val0->getType()) && (getRandom() % 1)) {
+    if (isa<FixedVectorType>(Val0->getType()) && (getRandom() & 1)) {
       unsigned NumElem =
           cast<FixedVectorType>(Val0->getType())->getNumElements();
       CondTy = FixedVectorType::get(CondTy, NumElem);
     }
 
     Value *Cond = getRandomValue(CondTy);
     Value *V = SelectInst::Create(Cond, Val0, Val1, "Sl", BB->getTerminator());
     return PT->push_back(V);