Warnings in operation counting

Hey!

When I count the operations of a kernel with Loo.py, I get a lot of warnings that say something similar to the following.
```
LoopyWarning: in kernel some_insns_knl: get_insn_count: when counting instruction out with count_granularity=subgroup, using upper bound for work-group size (1 work-items) to compute sub-groups per work-group. When multiple device programs present, actual sub-group count may be lower. (add 'insn_count_subgroups_upper_bound' to silenced_warnings kernel attribute to disable)
warn_with_kernel(knl, "insn_count_subgroups_upper_bound",
```
As adviced in the error message I am trying to silence the warnings with the following
```
knl = slate_wrapper_knl.default_entrypoint
warnings = list(knl.silenced_warnings)
warnings.extend(["insn_count_subgroups_upper_bound", "no_lid_found"])
knl = knl.copy(silenced_warnings=warnings)
slate_wrapper_knl = slate_wrapper_knl.with_kernel(knl)
op_map = lp.get_op_map(slate_wrapper_knl, subgroup_size='guess')
```
But the warnings are not shut up. Am I doing this wrong?
An MFE can be found here https://gist.github.com/sv2518/c0639e1b7704ac3bdbc994d632062cea.