[PPC64] Add support for clrbhrb, mfbhrbe, rfebb.

[PPC64] Add support for clrbhrb, mfbhrbe, rfebb.

This patch adds support for the ISA 2.07 additions involving the
branch history rolling buffer and event-based branching.  These will
not be used by typical applications, so built-in support is not
required.  They will only be available via inline assembly.

Assembly/disassembly tests are included in the patch.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238032 91177308-0d34-0410-b5e6-96231b3b80d8