microsoft/clc: Fix images that have multiple derefs for real

A followup to !26435 (merged). That prevented the crash, but was still causing garbage shaders to come out at the end, which didn't pass DXIL validation, because my local testing missed the validate step. I've managed to understand why opt_dead_cf was causing the problem, and reproduced a trivial example (thankfully CL -> SPIR-V is done at -O0 to preserve dumb unnecessary control flow).