[CorrelatedValuePropagation] Remove redundant if statement in processSelect()

This statement

    if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType());

is introduced in https://reviews.llvm.org/rG35609d97ae89b8e13f40f4e6b9b056954f8baa83
to fix a case where unreachable code can cause select instruction
simplification to fail. In https://reviews.llvm.org/rGd10480657527ffb44ea213460fb3676a6b1300aa,
we begin to perform a depth-first walk of basic blocks. This means
we will not visit unreachable blocks. So we do not need this the
special check any more.

Differential Revision: https://reviews.llvm.org/D76753

Author: Enna1

llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -125,19 +125,15 @@ Pass *llvm::createCorrelatedValuePropagationPass() {
 
 static bool processSelect(SelectInst *S, LazyValueInfo *LVI) {
   if (S->getType()->isVectorTy()) return false;
-  if (isa<Constant>(S->getOperand(0))) return false;
+  if (isa<Constant>(S->getCondition())) return false;
 
   Constant *C = LVI->getConstant(S->getCondition(), S->getParent(), S);
   if (!C) return false;
 
   ConstantInt *CI = dyn_cast<ConstantInt>(C);
   if (!CI) return false;
 
-  Value *ReplaceWith = S->getTrueValue();
-  Value *Other = S->getFalseValue();
-  if (!CI->isOne()) std::swap(ReplaceWith, Other);
-  if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType());
-
+  Value *ReplaceWith = CI->isOne() ? S->getTrueValue() : S->getFalseValue();
   S->replaceAllUsesWith(ReplaceWith);
   S->eraseFromParent();