[Lanai] fix MC / objdump

D78776 removed is{Call,Branch,UnconditionalBranch} guards in objdump
before calling MCInstrAnalysis::evaluateBranch. This is fine for other
architectures as they gracefully handle evaluateBranch being called on
non-branches. However, the Lanai MCInstrAnalysis implementation didn't
and that change caused it to crash.

This inserts the same guards back into Lanai's evaluateBranch
implementation and adds a smoke test that exercises `llc | objdump` so
this kind of regression is hopefully caught next time.

Reviewed By: jpienaar, MaskRay

Differential Revision: https://reviews.llvm.org/D107593