[llvm-c][Disassembler] When printing latency information, fall back to the
itinerary model in case the target does not supply a scheduling model.

By doing this, targets like cortex-a8 can benefit from the latency printing
feature added in r191859.

This part of <rdar://problem/14687488>. 


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@191916 91177308-0d34-0410-b5e6-96231b3b80d8