pow: Port from amd_builtins

Passes piglit on turks and carrizo
fp64 passes CTS on carrizo

v2: fix formatting
    check fp32 denormal support at runtime

Reviewer: Jeroen Ketema <j.ketema@xs4all.nl>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
llvm-svn: 322760