Delete bitset iterators defines (#12642)

Delete bitset iterators defines (#12642)

Delete BLOCKSET_ITER_INIT

Rename blkNum to bbNum.
We usually use bbNum (basic block number), rather than blkNum(block
number).
This change allows to grep for iterator and etc easier.

Delete BITVEC_ITER_INIT and BITVEC_INIT_NOCOPY.

Delete VARSET_ITER_INIT.

Rename some variables around varSet iterators.