Tighten the insert() API for bundled instructions.

The normal insert() function takes an MBB::iterator position, and
inserts a stand-alone MachineInstr as before.

The insert() function that takes an MBB::instr_iterator position can
insert instructions inside a bundle, and will now update the bundle
flags correctly when that happens.

When the insert position is between two bundles, it is unclear whether
the instruction should be appended to the previous bundle, prepended to
the next bundle, or stand on its own. The MBB::insert() function doesn't
bundle the instruction in that case, use the MIBundleBuilder class for
that.

llvm-svn: 170437