stream-op: set str for AST_BLOCK
stream-op: set str for AST_BLOCK

Signed-off-by: Kamil Rakoczy <539d6a2650324aa9156edbbb09228193dfee5b9b@antmicro.com>

SymbiFlow/yosys-f4pga-plugins,chipsalliance/yosys-f4pga-plugins,antmicro/yosys-symbiflow-plugins,antmicro/yosys-symbiflow-plugins,chipsalliance/yosys-f4pga-plugins,SymbiFlow/yosys-symbiflow-plugins,SymbiFlow/yosys-f4pga-plugins,SymbiFlow/yosys-f4pga-plugins,SymbiFlow/yosys-symbiflow-plugins,SymbiFlow/yosys-symbiflow-plugins,antmicro/yosys-symbiflow-plugins