[X86][3DNOW] Teach decoder about AMD 3DNow! instrs

Summary:
This patch makes the decoder understand old AMD 3DNow!
instructions that have never been properly supported in the X86
disassembler, despite being supported in other subsystems. Hopefully
this should make the X86 decoder more complete with respect to binaries
containing legacy code.

Reviewers: craig.topper

Reviewed By: craig.topper

Subscribers: llvm-commits, maksfb, bruno

Differential Revision: https://reviews.llvm.org/D43311

llvm-svn: 325295