18.104.22.168: more conservative global variable conversion
Based on patch by Roman Marynchak.
* Modify IR1-CONVERT-VAR to emit SYMBOL-VALUE wrapper
for all global variables except those which are
* 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