LegalizeTypes support for promotion of bswap.

In LegalizeDAG the value is zero-extended to
the new type before byte swapping.  It doesn't
matter how the extension is done since the new
bits are shifted off anyway after the swap, so
extend by any old rubbish bits.  This results
in the final assembler for the testcase being
one line shorter.

llvm-svn: 53604