Some first bits of AltiVec stuff: Instruction Formats, Encodings, and

Registers.  Apologies to Jim if the scheduling info so far isn't accurate.

There's a few more things like VRsave support that need to be finished up
in my local tree before I can commit code that Does The Right Thing for
turning 4 x float into the various altivec packed float instructions.

llvm-svn: 24489