LinAlgMatrix metadata cleanup

[hekota]

For every attributed LinAlgMatrix type used in the source code the compiler creates an entry under !dx.targetTypes metadata node. After optimization this list should be pruned to contain only types that are actually used in the module. This should probably happen in DxilPreparePasses. This should also apply when linking multiple modules compiled with lib_6_x.

Also, since the named metadata node !dx.targetTypes is created early on during CodeGen, it becomes the first named metadata node in the module. So it shows up before the mandatory nodes like as !dx.version or !dx.shaderModel. While the order of named metadata nodes is not very improtant, it is probably a good idea to move it after the mandatory nodes.