gallium/llvm: implement geometry shaders in the llvm paths

gallium/llvm: implement geometry shaders in the llvm paths

This commits implements code generation of the geometry shaders in
the SOA paths. All the code is there but bugs are likely present.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>