dylan: Fix return type of a subtype? method

* sources/dylan/type.dylan
  (subtype? on <limited-collection-type> and <class>): Fix return
   value declaration to permit #t.
Bug 7458: Fix primitive-c-*-at{,-setter}

* sources/dfmc/llvm-back-end/llvm-primitives-ffi.dylan
  (raw-accessor-primitive-definer): Properly use both offset and byte-offset
   in the computation of the pointer address.
Bug 7458: Fix dependencies on dead code

Generate values for referenced temporaries generated by dead
code (following loops, etc.)

* sources/dfmc/llvm-back-end/llvm-emit-computation.dylan
  (emit-dead-assignment on <temporary>,
   emit-dead-assignment on <multiple-value-temporary>): New methods
   to assign a dummy value to the generated temporary.
  (emit-computations): Assign dummy values using emit-dead-assignment
   when the emitted computation does not fall within a valid basic
   block.