1.0.48.14: more conservative global variable conversion

1.0.48.14: more conservative global variable conversion

 Based on patch by Roman Marynchak.

 Fixes lp#722734.

 * Modify IR1-CONVERT-VAR to emit SYMBOL-VALUE wrapper
   for all global variables except those which are
   ALWAYS-BOUND.

 * Modify IR1-STEP-FORM-P to return false for SYMBOL-VALUE
   with constant argument, now that virtually all global
   variables are accessed with a function call (which gets
   converted via a VOP, so the final machine code remains
   the same.)