Allow explicit BYVAL in FUNCTION= or RETURN when returning BYREF
Fix missing implicit DEREF for BYREF-proc calls as lvalues
Add tiny examples for BYREF results and VIRTUALs
LLVM backend: Fix invalid IRVREG.mult access in assert()

(the mult field is only valid for IDX and PTR vregs, not for VARs,
this could be accessing uninitialized memory)
C/LLVM backends: Don't bother setting IRVREG.mult, it's unused anyways