Clean out per-function data after the machine code verifier is done with it.

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