Remove target edge list from IceInstBr.

This was duplicating the information already stored in the enclosing
IceCfgNode.  Keeping it in IceCfgNode will make it easier to split a
node, otherwise splitting would require locating all branch
instructions in the block (lowering a 64-bit compare/branch into
32-bit operations may result in 2 sets of 32-bit compare/branch), and
updating their targets as well.