[hwasan] [nfc] simplify getAllocaSizeInBytes

[fmayer]

Summary

AllocaInst::getAllocationSize implements essentially the same logic as
our custom function.

llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp

IRB.CreateCall(HwasanMemoryAccessCallbackSized[O.IsWrite],
                     ConstantInt::get(IntptrTy, O.TypeSize / 8)});
   }
   untagPointerOperand(O.getInsn(), Addr);
 
   return true;
 }
 
 static uint64_t getAllocaSizeInBytes(const AllocaInst &AI) {
-  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();
-  uint64_t SizeInBytes = AI.getModule()->getDataLayout().getTypeAllocSize(Ty);
-  return SizeInBytes * ArraySize;
+  auto DL = AI.getModule()->getDataLayout();
+  return AI.getAllocationSizeInBits(DL).getValue() / 8;
 }
 
 void HWAddressSanitizer::tagAlloca(IRBuilder<> &IRB, AllocaInst *AI, Value *Tag,
                                    size_t Size) {
   size_t AlignedSize = alignTo(Size, Mapping.getObjectAlignment());
   if (!UseShortGranules)
     Size = AlignedSize;