[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.

GitOrigin-RevId: 02eb38119eda7c060a6b96a1169fd2b98992b617
1 file changed