Added a FIXNUM type of register analagous to ADDRESS registers.

Added a FIXNUM type of register analagous to ADDRESS registers.
Their current use is to hold the results of OBJECT->FIXNUM expressions
and assignments of such registers to other registers. We have to tell
the cse code that such registers and expressions are invalid across
anything that can call the garbage collector. This is because the
garbage collector would currently barf on such values that have no
type codes (i.e. fixnums which have had their type codes removed).
In the future, FIXNUM registers may contain the reults of fixnum
arithmetic expressions, so that we are not doing so much boxing and
unboxing. Currently however, all such results are boxed into their
destinations.