Add bitcode compatibility test for 6.0

Summary:
Add bitcode compatibility test for 6.0. On top of the normal disassemble
test, also runs the verifier to make sure simple 6.0 bitcode can pass
the current IR verifier.

Reviewers: vsk

Reviewed By: vsk

Subscribers: dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D49086

llvm-svn: 336574