Use bidirectional bundle flags to simplify important functions.

The bundle_iterator::operator++ function now doesn't need to dig out the
basic block and check against end(). It can use the isBundledWithSucc()
flag to find the last bundled instruction safely.

Similarly, MachineInstr::isBundled() no longer needs to look at
iterators etc. It only has to look at flags.

llvm-svn: 170473