Several major changes:

1. Pull out the expand cases for BSWAP and CT* into a separate function,
   reducing the size of LegalizeOp.
2. Fix a bug where expand(bswap i64) was wrong when i64 is legal.
3. Changed LegalizeOp/PromoteOp so that the legalizer never needs to be
   iterative.  It now operates in a single pass over the nodes.
4. Simplify a LOT of code, with a net reduction of ~280 lines.

llvm-svn: 25736