make upgraded-complex-part-type less clever

Unfortunately the requirements under CLHS TYPEP constrain the
implementation of UPGRADED-COMPLEX-PART-TYPE severely; the
requirement that all others of the same specialized representation
satisfy the typep requirements individually and severally on real
and imaginary parts means (we believe) that the only possible
implementation of UPGRADED-COMPLEX-PART-TYPE is effectively the
trivial implementation.  Make it so.

src/compiler/generic/vm-type.lisp

@@ -144,19 +144,9 @@
  ((eq type *empty-type*) nil)
  ((not type) (error "Undefined type: ~S" spec))
  (t
- (let ((ctype (specifier-type `(complex ,spec))))
- (cond
- ((eq ctype *empty-type*) '(eql 0))
- ((csubtypep ctype (specifier-type '(complex single-float)))
- 'single-float)
- ((csubtypep ctype (specifier-type '(complex double-float)))
- 'double-float)
- #!+long-float
- ((csubtypep ctype (specifier-type '(complex long-float)))
- 'long-float)
- ((csubtypep ctype (specifier-type '(complex rational)))
- 'rational)
- (t 'real)))))))
+ (let ((ctype (specifier-type `(complex ,spec)))) ; error checking
+ (declare (ignore ctype))
+ (type-specifier type))))))
 
 ;;; Return the most specific integer type that can be quickly checked that
 ;;; includes the given type.