DBG-REDUCE now stored %stack-closure-ref (and also %read-closure-ref)

DBG-REDUCE now stored %stack-closure-ref (and also %read-closure-ref)
expressions in a compact form as this is the dominant expression kind.

CLOSCONV generates these expressions directly, and DBG-INFO/REMEMBER
converts others.  STACKOPT is modified to update the stack frame
ordering and INDEXIFY leaves them alone (the call to vectro-index
occurs in DBG info generation).