[Inline] Add test for exponential deferred inlining (NFC)

This shows a case where deferred inlining produces an exponential
result. The test case demonstrates the basic exponential behavior,
but is nowhere close to the worst case. For example, the file at
https://gist.github.com/nikic/1262b5f7d27278e1b34a190ae10947f5
currently gets expanded from <100 lines to nearly 500000 lines of
IR by opt -inline.

Author: nikic