[libclc] Fix out-of-bound value for workitem functions according to OpenCL spec

libclc/clc/lib/amdgcn/workitem/clc_get_group_id.cl

@@ -17,6 +17,6 @@ _CLC_DEF _CLC_OVERLOAD size_t __clc_get_group_id(uint dim) {
   case 2:
     return __builtin_amdgcn_workgroup_id_z();
   default:
-    return 1;
+    return 0;
   }
 }

libclc/clc/lib/amdgcn/workitem/clc_get_local_id.cl

@@ -17,6 +17,6 @@ _CLC_DEF _CLC_OVERLOAD size_t __clc_get_local_id(uint dim) {
   case 2:
     return __builtin_amdgcn_workitem_id_z();
   default:
-    return 1;
+    return 0;
   }
 }

libclc/clc/lib/ptx-nvidiacl/workitem/clc_get_local_size.cl

@@ -17,6 +17,6 @@ _CLC_OVERLOAD _CLC_DEF size_t __clc_get_local_size(uint dim) {
   case 2:
     return __nvvm_read_ptx_sreg_ntid_z();
   default:
-    return 0;
+    return 1;
   }
 }

libclc/clc/lib/ptx-nvidiacl/workitem/clc_get_num_groups.cl

@@ -17,6 +17,6 @@ _CLC_OVERLOAD _CLC_DEF size_t __clc_get_num_groups(uint dim) {
   case 2:
     return __nvvm_read_ptx_sreg_nctaid_z();
   default:
-    return 0;
+    return 1;
   }
 }