nir: Handle divergence for decl_reg

nir: Handle divergence for decl_reg

Once decl_reg is handled, src[0].ssa->divergent will be properly set, so
load_reg and load_reg_indirect do not need special treatment.

shader-db can run to completion on HSW, IVB, and SNB now. No other
testing was done.

v2: Refactor nir_intrinsic_load_reg and nir_intrinsic_load_reg_indirect
handling. Suggested by Daniel Schürmann.

Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Fixes: 4fd257d20fe ("nir: Properly handle divergence for load_reg")
Fixes: 6dbb5f1e07b ("intel/fs: rerun divergence analysis prior to convert_from_ssa")
Closes: #10233
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26436>
(cherry picked from commit 7fce0a5598fbfba73e10460b37865fe54b7c5bfc)