R600: Enable vector fpow.

The OpenCL specs say: "The vector versions of the math functions operate
component-wise. The description is per-component."

Patch by: Jan Vesely

Reviewed-by: Tom Stellard <thomas.stellard@amd.com>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 200773