relational: Implement signbit

v2 Changes:
   - use __builtin_signbit instead of shifting by hand
   - significantly improve vector shuffling
   - Works correctly now for signbit(float16) on radeonsi

Signed-off-by: Aaron Watry <awatry@gmail.com>
Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
llvm-svn: 211696