[AArch64] Performance enhancements for Cavium ThunderX2 T99

This patch enables significant performance enhancements to the
Cavium ThunderX2T99 LLVM backend, as observed by running SPEC2K6,
by adding more detailed scheduling information.

Related Bugzilla bug: http://bugs.llvm.org/show_bug.cgi?id=32562

Patch by: steleman

Differential Revision: https://reviews.llvm.org/D31801

llvm-svn: 306462