Fix a problem with the reverse bundle iterators.

Fix a problem with the reverse bundle iterators.

This showed up the first time rend() was called on a bundled instruction
in the Mips backend.

Also avoid dereferencing end() in bundle_iterator::operator++().

We still don't have a place to put unit tests for this stuff.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158310 91177308-0d34-0410-b5e6-96231b3b80d8