DSLX: Check for send/receive validity

[RobSpringer]

On more than one occasion, I've written invalid DSLX that sends on an individual channel more than once per iteration. There's probably other stuff that should be enforced that I can't think of immediately. Before getting to IR conversion, we should do a pass to make sure that we're not improperly using channels.

[hongted]

For this check, there will probably need a looser spec for procs used only as a test driver and a stricter one for IR conversion.