kernel lacks LocalSize execution mode if another kernel has reqd_work_group_size

Here the "align" kernel has a reqd_work_group_size. So it gets a LocalSize execution mode. It also suppresses generation of spec IDs for the components of a work-group-size vector.
However, the "boo" kernel is generated without an associated LocalSize execution model. This is an error. The most sane thing to do here is to emit a LocalSize of 1 1 1, and document it.

```
kernel void __attribute__((reqd_work_group_size(12,2,3))) align(global int* A, int x, float4 c) {
*A = x + (int)c.x;
}

kernel void boo(global int* A, int x, float4 c) {
*A = x + (int)c.x;
}
```

There's an unfortunate problem in the Vulkan env spec in that a specialization value for workgroup size is freefloating and you can't tell what compute shaders it should affect. So the proposed defaulting is about as good as we can do.