Fix the build on OS X 10.8.0

It seems our exception handler can be called before it's fully set up.
Handle that case without potentially leaking too many ports.

Reported by Gabriel Dos Reis on sbcl-devel.
Substitute constants with modular equivalents more safely

* Modular arithmetic sometimes lets us narrow constants down,
  especially with signed arithmetic. We now update the receiving
  LVAR's type conservatively when there are multiple uses; otherwise,
  conflicting type information results in spurious dead code
  elimination.

* Test case by Eric Marsden.

* Reported by Eric Marsden on sbcl-devel (2013-04-18).
Trivial code cleanups

 Declare a variable as ignored, and descriptors are 64 bit on
 x86-64. The latter was brought to my attention by Douglas Katzman.