[SROA] NFC: Look at TypeStoreSize scalable property, rather than at type directly.

Some places in the code have checks for isa<ScalableVectorType> and use
that to bail out of the code. It's also possible to look directly at the
allocated type-size and check if the size is scalable. This means it's
possible to also support other scalable types that are not vectors (i.e.
TargetExtType).

This is split out from D136861.