vkd3d-shader/ir: Allow failure in shader_signature_find_element_for_reg().

shader_signature_find_element_for_reg() is also used in the TPF parser, where the program has not been validated yet, so it must not crash on errors.

The I/O normaliser can instead assume that the shader is already validated.

This fixes a crash with a shader used by The Falconeer. The bug is still present, because the shader will be incorrectly rejected, but at least the vkd3d-shader will fail gracefully.