[NFC] Rename variables to FPOp (#173792)

In my earlier PR (https://github.com/llvm/llvm-project/pull/167574),
I've named a variable in fpext function wrong. I've changed the name in
both functions to generic FPOp

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
index 99d31cd..d570581 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -3997,8 +3997,8 @@
   SDValue N = getValue(I.getOperand(0));
   SDLoc dl = getCurSDLoc();
   SDNodeFlags Flags;
-  if (auto *TruncInst = dyn_cast<FPMathOperator>(&I))
-    Flags.copyFMF(*TruncInst);
+  if (auto *FPOp = dyn_cast<FPMathOperator>(&I))
+    Flags.copyFMF(*FPOp);
   const TargetLowering &TLI = DAG.getTargetLoweringInfo();
   EVT DestVT = TLI.getValueType(DAG.getDataLayout(), I.getType());
   setValue(&I, DAG.getNode(ISD::FP_ROUND, dl, DestVT, N,
@@ -4013,8 +4013,8 @@
   EVT DestVT = DAG.getTargetLoweringInfo().getValueType(DAG.getDataLayout(),
                                                         I.getType());
   SDNodeFlags Flags;
-  if (auto *TruncInst = dyn_cast<FPMathOperator>(&I))
-    Flags.copyFMF(*TruncInst);
+  if (auto *FPOp = dyn_cast<FPMathOperator>(&I))
+    Flags.copyFMF(*FPOp);
   setValue(&I, DAG.getNode(ISD::FP_EXTEND, getCurSDLoc(), DestVT, N, Flags));
 }