DAGCombiner: Fix broken size check in isAlias

This should have been converting the size to bytes, but wasn't really.
These should probably all be using getStoreSize instead.

I haven't been able to come up with a meaningful testcase for this.
I can trigger it using combinations of struct loads and stores,
but can't observe a difference in non-broken testcases.

isAlias is only really used during store merging, so I'm not sure how
to get into the vector splitting situation the comment describes
since store merging is only done before type legalization.

llvm-svn: 271356