vkd3d-shader/ir: Do not crash when registers cannot be resolved to signature elements.
Since it's not clear which direction !1548 is going to take, in the meantime I'd like to at least address the crash and report it as a compiler error instead. With this patch no D3DBC shader in my collection causes a crash when compiling to SPIR-V, though a lot of them will fail compilation anyway (some of them due to the problem discussed in !1548, that still need to be addressed).