spirv: Don't leak GS initialization to other stages

The stage specific fields of shader_info are in an union.  We've
likely been lucky that this value was either overwritten or ignored by
other stages.  The recent change in shader_info layout in commit
84a1a2578d ("compiler: pack shader_info from 160 bytes to 96 bytes")
made this issue visible.

Fixes: cf2257069c ("nir/spirv: Set a default number of invocations for geometry shaders")
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/compiler/spirv/spirv_to_nir.c

@@ -5152,7 +5152,8 @@ spirv_to_nir(const uint32_t *words, size_t word_count,
    }
 
    /* Set shader info defaults */
-   b->shader->info.gs.invocations = 1;
+   if (stage == MESA_SHADER_GEOMETRY)
+      b->shader->info.gs.invocations = 1;
 
    /* Parse rounding mode execution modes. This has to happen earlier than
     * other changes in the execution modes since they can affect, for example,