[cfi-verify] Add an interesting unit test where undef search length changes result.

Add an interesting unit test, found by changing --search-length-undef from the default. Program handles it correctly but good for ensuring correctness on further changes :)

Reviewers: pcc

Subscribers: mgorny, llvm-commits, kcc, vlad.tsyrklevich

Differential Revision: https://reviews.llvm.org/D38658

llvm-svn: 317355