Right now, cproc can segfault if it tries to call a function with a long double argument. We currently detect a missing representation when we try to emit it, but we should actually just prevent construction of a value with no QBE representation in the first place.

Another manifestation of this bug is if you try to call a function returning long double, ignoring the result. This should be a compile error, but cproc compiles it as if it were void.

funcinst currently uses repr == NULL for function calls to indicate that it has no return value, so we can't tell if we are calling a function with no return value, or the return value has no representation.

To fix this, perhaps we could add a special funccall function to create call instructions. There is already some call-specific code in funcinst, so this is probably the right thing to do.