[IVDescriptors] Call getOpcode on demand in getReductionOpChain. nfc (#118777)

Non-arithmetic reductions do not require the binary opcodes.
As a first step toward removing the dependency of non-arithmetic
reductions on `getOpcode` function, this patch refactors the
`getReductionOpChain` function.

In the future, once all users of `getOpcode` function are refactored, an
assertion can be added to `getOpcode` function to ensure that only
arithmetic reductions rely on it.

Author: Mel-Chen

llvm/lib/Analysis/IVDescriptors.cpp

@@ -1189,7 +1189,6 @@ unsigned RecurrenceDescriptor::getOpcode(RecurKind Kind) {
 SmallVector<Instruction *, 4>
 RecurrenceDescriptor::getReductionOpChain(PHINode *Phi, Loop *L) const {
   SmallVector<Instruction *, 4> ReductionOperations;
-  unsigned RedOp = getOpcode();
   const bool IsMinMax = isMinMaxRecurrenceKind(Kind);
 
   // Search down from the Phi to the LoopExitInstr, looking for instructions
@@ -1237,7 +1236,7 @@ RecurrenceDescriptor::getReductionOpChain(PHINode *Phi, Loop *L) const {
     if (isFMulAddIntrinsic(Cur))
       return true;
 
-    return Cur->getOpcode() == RedOp;
+    return Cur->getOpcode() == getOpcode();
   };
 
   // Attempt to look through Phis which are part of the reduction chain