[SDAG] Allow Unknown sizes when refining MMO alignments. NFC

The changes in D113888 / 32b6c17 altered the memory size of a
masked store, as it will store an unknown number of bytes not the full
vector size. We can have situations where the masked stores is legalized
and then turned to a normal store, as the mask is known to be all ones.
This creates a store with an unknown size MMO that was hitting this
assert.

The store created can be given a better size in a followup patch. This
currently adjusts the assert to handle unknown sizes.

llvm/lib/CodeGen/MachineOperand.cpp

@@ -1071,7 +1071,9 @@ void MachineMemOperand::refineAlignment(const MachineMemOperand *MMO) {
   // The Value and Offset may differ due to CSE. But the flags and size
   // should be the same.
   assert(MMO->getFlags() == getFlags() && "Flags mismatch!");
-  assert(MMO->getSize() == getSize() && "Size mismatch!");
+  assert((MMO->getSize() == ~UINT64_C(0) || getSize() == ~UINT64_C(0) ||
+          MMO->getSize() == getSize()) &&
+         "Size mismatch!");
 
   if (MMO->getBaseAlign() >= getBaseAlign()) {
     // Update the alignment value.