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
84a1a257 ("compiler: pack shader_info from 160 bytes to 96 bytes")
made this issue visible.

Fixes: cf225706 ("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>
(cherry picked from commit 087ecd9c)