spirv: workaround LLVM-SPIRV Undef variable initializers

spirv: workaround LLVM-SPIRV Undef variable initializers

The LLVM-SPIRV translator creates variables with initializers, but
most of those are actually undef initializers. We can just skip
composites that are entirely made of undefs, but for partially undefs,
we will still zero initialize.

v2: Rename wa_llvm_spirv_undef_initializer to wa_llvm_spirv_ignore_workgroup_initializer (Caio)
    Limit workaround to OpenCL (Caio)
    Make workaround clearer (Caio)

v3: Only apply workaround on workgroup storage (Caio)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13030>