[x86] translate SSE packed FP comparison builtins to IR

As noted in the code comment, a potential follow-on would be to remove
the builtins themselves. Other than ord/unord, this already works as 
expected. Eg:

  typedef float v4sf __attribute__((__vector_size__(16)));
  v4sf fcmpgt(v4sf a, v4sf b) { return a > b; }

Differential Revision: http://reviews.llvm.org/D21268

llvm-svn: 272840