[ASAN] Use AI.getAllocationSize() helper (NFC)

Author: nikic

llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp

@@ -668,16 +668,7 @@ struct AddressSanitizer {
   }
 
   TypeSize getAllocaSizeInBytes(const AllocaInst &AI) const {
-    uint64_t ArraySize = 1;
-    if (AI.isArrayAllocation()) {
-      const ConstantInt *CI = dyn_cast<ConstantInt>(AI.getArraySize());
-      assert(CI && "non-constant array size");
-      ArraySize = CI->getZExtValue();
-    }
-    Type *Ty = AI.getAllocatedType();
-    TypeSize SizeInBytes =
-        AI.getModule()->getDataLayout().getTypeAllocSize(Ty);
-    return SizeInBytes * ArraySize;
+    return *AI.getAllocationSize(AI.getModule()->getDataLayout());
   }
 
   /// Check if we want (and can) handle this alloca.