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

The changes in D113888 / 32b6c17b 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.