Also don't dereference old pointers after they have been deleted causing
random crashes when enabling the machine code verifier.

Ahem...

I have not included a test case for the crash. It hapened when enabling the
verifier on CodeGen/X86/2009-08-06-branchfolder-crash.ll.

The crash depends on an MBB being allocated at the same address as a
previously deleted MBB. I don't think that can be reproduced reliably.

llvm-svn: 78472