Implement support for the bizarre 3DNow! encoding (which is unlike anything

Implement support for the bizarre 3DNow! encoding (which is unlike anything
else in X86), and add support for pavgusb.  This is apparently the
only instruction (other than movsx) that is preventing ffmpeg from building
with clang.

If someone else is interested in banging out the rest of the 3DNow!
instructions, it should be quite easy now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@115466 91177308-0d34-0410-b5e6-96231b3b80d8