[NFC][AMDGPU] Fix unused-variable warning (llvm#81040)

This is only used in assert statement.

Author: pranavk

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -12016,7 +12016,7 @@ static SDValue getDWordFromOffset(SelectionDAG &DAG, SDLoc SL, SDValue Src,
 
 static SDValue matchPERM(SDNode *N, TargetLowering::DAGCombinerInfo &DCI) {
   SelectionDAG &DAG = DCI.DAG;
-  EVT VT = N->getValueType(0);
+  [[maybe_unused]] EVT VT = N->getValueType(0);
   SmallVector<ByteProvider<SDValue>, 8> PermNodes;
 
   // VT is known to be MVT::i32, so we need to provide 4 bytes.