[VE] Convert test to opaque pointers (NFC)

There is a minor codegen regression here (an extra and instruction).
The reason is that CGP only eliminates fallthrough branches if it
has made some other kind of change, and with opaque pointers that
other change does not occur.

Ideally, we should probably always try to eliminate fallthroughs,
but this runs into the problem that performing a dummy fallthrough
is a common pattern in tests for forcing SDAG to select them
separately, so it's not quite that simple.