Merge concepts of `address' and `fixnum' register into `non-object'

Merge concepts of `address' and `fixnum' register into `non-object'
register.  Implement operations to detect substitutable register
within a given expression, and to substitute subexpressions for those
registers.  Implement predicate to determine if an expression is
constant.  All of these new operations are used by the improved
register combiner.

Simplify `rtl:trivial-expression?' by disallowing stack references.
This causes some inefficiencies that must be corrected elsewhere, but
reveals more intermediate values to the CSE.