nir: Handle divergence for decl_reg
shader-db can run to completion on HSW, IVB, and SNB now. No other testing was done.
Fixes: 4fd257d2 ("nir: Properly handle divergence for load_reg")
Fixes: 6dbb5f1e ("intel/fs: rerun divergence analysis prior to convert_from_ssa")
Closes: #10233 (closed)