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 <f17cb7083eb6a94c3fb9d8f09ceefa518f721ab2@intel.com>
Reviewed-by: Yang, Rong R <78759e37dc7114caed18f74e7515c5cb519dbf6d@intel.com>

zhenyw/beignet,wdv4758h/beignet,wdv4758h/beignet,freedesktop-unofficial-mirror/beignet,ignatenkobrain/beignet,freedesktop-unofficial-mirror/beignet,ignatenkobrain/beignet,zhenyw/beignet,wdv4758h/beignet,freedesktop-unofficial-mirror/beignet,wdv4758h/beignet,zhenyw/beignet,ignatenkobrain/beignet,ignatenkobrain/beignet,freedesktop-unofficial-mirror/beignet,zhenyw/beignet,ignatenkobrain/beignet,zhenyw/beignet,wdv4758h/beignet