This works around an issue introduced by commit
66cb2815f0. SV_PRIMITIVE_ID inputs in
geometry shaders use VKD3DSPR_PRIMID registers, and we create the
corresponding SPIR-V inputs using spirv_compiler_emit_io_register().
Unfortunately we also have an input signature element for the same
input, and simply creating another PrimitiveId input would run into
VUID-StandaloneSpirv-OpEntryPoint-09658.
Before the commit mentioned above, we'd use DCL_INPUT instructions to
emit input declarations, and these would help to distinguish whether
VKD3DSPR_INPUT or VKD3DSPR_PRIMID registers were used for primitive ID
inputs. Note that we can't simply ignore input signature element with
SIGNATURE_TARGET_LOCATION_UNUSED; the DXIL parser emits SV_SAMPLE_INDEX
inputs with that target location, but does require them to use a
VKD3DSPR_INPUT register.
The change in vsir_program_iterator_next() is necessary to allow us to
introduce instructions before the iterator using:
vsir_program_iterator_prev(&it);
vsir_program_iterator_inset_after(&it, n);
vsir_program_iterator_next(&it);
This since (it.idx == SIZE_MAX) is equivalent to the iterator being
before the beginning of the list.
