aco: zero-initialize Temp

Fixes dEQP-VK.transform_feedback.* crashes from accesses garbage
temporaries in emit_extract_vector().

Fixes: 85521061 ("aco: prepare helper functions for subdword handling")
Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4463>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4463>

src/amd/compiler/aco_optimizer.cpp

@@ -103,6 +103,8 @@ struct ssa_info {
    };
    uint32_t label;
 
+   ssa_info() : label(0) {}
+
    void add_label(Label new_label)
    {
       /* Since all labels which use "instr" use it for the same thing