Add a 'count' variable to each symbol which gets increased every time
the symbol is referenced.  And then modify the register definition to
include counts for symbols which are referenced from the source code
only and not from the sequencer code.

This will give us an automatic usage count for the symbols with only
minimal hand-crafting.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>