Speed up UPPER-CASE-P and LOWER-CASE-P.

Avoid calling MISC-INDEX twice.
equal .lisp-obj files produce identical cold-sbcl.cores

(apart from the build-id, that is.  But now
  cmp -i 1024 {sbcl,$X}/output/cold-sbcl.core
returns true for X in {ccl32,ccl64,clisp})