select: simplify implementation and fix fp16

Fix half precision implementation
Vector ?: operator should behave exactly as select
Passes CTS on carrizo

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewer: Jeroen Ketema <j.ketema@xs4all.nl>

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@329462 91177308-0d34-0410-b5e6-96231b3b80d8