LegalizeDAG: Don't replace vector store with integer if not legal

LegalizeDAG: Don't replace vector store with integer if not legal

For the same reason as the corresponding load change.

Note that ExpandStore is completely broken for non-byte sized element
vector stores, but preserve the current broken behavior which has tests
for it. The behavior should be the same, but now introduces a new typed
store that is incorrectly split later rather than doing it directly.

llvm-svn: 264928