The layout for struct block_loop under ithreads can be simplified.

The layout for struct block_loop under ithreads can be simplified.
Instead of wedging the pad offset into a void* iterdata, and always
storing PL_comppad even when it isn't used, instead do this:

PAD        *oldcomppad; /* Also used for the GV, if targoffset is 0 */
/* This is also accesible via cx->blk_loop.my_op->op_targ */
PADOFFSET  targoffset;

and store the GV pointer in oldcompad. Pointers to pointers seems
cleaner. This also allows us to eliminate the flag bit CXp_PADVAR.

p4raw-id: //depot/perl@33081