[SCEVExpander] Remove unused variable [NFC]

gcc warned about it:
 ../lib/Transforms/Utils/ScalarEvolutionExpander.cpp: In lambda function:
 ../lib/Transforms/Utils/ScalarEvolutionExpander.cpp:2104:22: warning: unused variable 'ARPtrTy' [-Wunused-variable]
  2104 |     if (PointerType *ARPtrTy = dyn_cast<PointerType>(ARTy)) {
       |                      ^~~~~~~
Fix the warning by removing the variable and turn dyn_cast into isa.