Necessary changes to codegen cttz efficiently on PowerPC

1. Teach LegalizeDAG how to better legalize CTTZ if the target doesn't have
   CTPOP, but does have CTLZ
2. Teach PPC32 how to do sub x, const -> add x, -const for valid consts
3. Teach PPC32 how to do and (xor a, -1) b -> andc b, a
4. Teach PPC32 that ISD::CTLZ -> PPC::CNTLZW

llvm-svn: 21880