fix a powr function issue in cpu compiler math
fix a powr function issue in cpu compiler math

In OpenCL spec, gentype powr(gentype x, gentype y). In the meantime,
added edge tests for powr.

Signed-off-by: Mengmeng meng <c9e0ecea92e14a35e72e26c7583f18422f51dcc3@intel.com>
Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet