[Lanai] Rename LanaiInstPrinter library to LanaiAsmPrinter

Summary:
    This is in keeping with LLVM convention. The classes are InstPrinters, but the library is ${target}AsmPrinter.

This patch is in response to bryant pointing out to me that Lanai was the only backend deviating from convention here. Thanks!

Reviewers: jpienaar, bryant

Subscribers: mgorny, jgosnell, llvm-commits

Differential Revision: https://reviews.llvm.org/D29043

llvm-svn: 292953