nir: fix NIR_DEBUG=validate_ssa_dominance

validate_ssa_def_dominance() asserts :

validate_assert(state, !BITSET_TEST(state->ssa_defs_found, def->index));

Because the previous validation lefts bits set when it processed the IR.

Signed-off-by: Lionel Landwerlin lionel.g.landwerlin@intel.com