Remove spir half vloada_half tests as this builtin does not exist in OpenCL (KhronosGroup#2552)

The test was using vloada_half which does not exist for scalars.

Also removed the files test.vloada_half_*.* from half.zip.

For test.vloada_half3_global, changed the OpenCL kernel to use
vload_half instead of vloada_half.

Build failures will return a proper failure now, before, the test was
passing in this case.

More Info: KhronosGroup/OpenCL-Docs#648

Author: AhmedAmraniAkdi