gallium/util: Fix leak in the live shader cache
When the nir backend is used, the create_shader call is supposed to release state->ir.nir. When the cache hits, create_shader is not called, thus state->ir.nir should be freed.
There is nothing to be done for the TGSI case as the tokens release is done by the caller.
This fixes a leak noticed in: #2931 (closed)
I'm not proposing stable for this patch as I'm not confident enough the patch would not cause possible regressions. However I can add it if some people think it should be fine (and please tell me how, I'm not used to the new system).