IR: Have byref imply dereferenceable

The langref already states it does, but this wasn't implemented. Also
covers inalloca and preallocated. Also helps fix a dependence on
pointer element types.

llvm/lib/IR/Value.cpp

@@ -712,11 +712,16 @@ uint64_t Value::getPointerDereferenceableBytes(const DataLayout &DL,
   CanBeNull = false;
   if (const Argument *A = dyn_cast<Argument>(this)) {
     DerefBytes = A->getDereferenceableBytes();
-    if (DerefBytes == 0 && (A->hasByValAttr() || A->hasStructRetAttr())) {
-      Type *PT = cast<PointerType>(A->getType())->getElementType();
-      if (PT->isSized())
-        DerefBytes = DL.getTypeStoreSize(PT).getKnownMinSize();
+    if (DerefBytes == 0) {
+      // Handle byval/byref/inalloca/preallocated arguments
+      if (Type *ArgMemTy = A->getPointeeInMemoryValueType()) {
+        if (ArgMemTy->isSized()) {
+          // FIXME: Why isn't this the type alloc size?
+          DerefBytes = DL.getTypeStoreSize(ArgMemTy).getKnownMinSize();
+        }
+      }
     }
+
     if (DerefBytes == 0) {
       DerefBytes = A->getDereferenceableOrNullBytes();
       CanBeNull = true;