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>

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@211696 91177308-0d34-0410-b5e6-96231b3b80d8