Don't require BUNDLE headers in MachineInstr::getBundleSize().

It is possible to build MI bundles that don't begin with a BUNDLE
header. Add support for such bundles, counting all instructions inside
the bundle.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171985 91177308-0d34-0410-b5e6-96231b3b80d8