Fix CSESubAccesses for SubAccesses with flips (#2112)

The flow of a LHS SubAccess node may still be SourceFlow if the type of
the Vec element has a flip. Tweak the logic of CSESubAccesses to check
every Expression flow while recursing instead of just the flow of the
final SubAccess.

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>