[AttributorAttributes] Remove unused variable [NFC]

gcc warned about it:
 [232/4788] Building CXX object lib/Transforms/IPO/CMakeFiles/LLVMipo.dir/AttributorAttributes.cpp.o
 ../lib/Transforms/IPO/AttributorAttributes.cpp: In lambda function:
 ../lib/Transforms/IPO/AttributorAttributes.cpp:12555:17: warning: unused variable 'SI' [-Wunused-variable]
 12555 |       if (auto *SI = dyn_cast<StoreInst>(Inst)) {
       |                 ^~

Fix the warning by removing the variable and turn dyn_cast into isa.