support const indexed global constant array

support const indexed global constant array

support reading global constant arrays by CONST index.

example:
  constant int o[3] = {71, 72, 73};

  kernel void f(global int *dst) {
    dst[get_global_id(0)] = o[2]; // const index: 2
  }

in llvm converting phase, calculate offset from const index,
then add the offset to array head

Signed-off-by: Homer Hsing <homer.xing@intel.com>
Reviewed-by: Yang, Rong R <rong.r.yang@intel.com>