x86_32 ABI: Don't try and expand structures with bitfields.

 - This is an ABI incompatiblity, but this is not likely to be a huge
   deal in practice. For now we at least generate self consistent code
   instead of crashing.
 - <rdar://problem/6657601> x86-32 ABI: Bitfields in small structures
   are not passed correctly

llvm-svn: 66713