Hardware testing revealed that SSY and PBK push to a different stack,
allowing code like this:
SSY label1;
PBK label2;
SYNC;
label1: PBK;
label2: EXIT;
Analysis passes do not have a good reason to depend on shader_ir.h to
work on top of nodes. This splits node-related declarations to their own
file and leaves the IR in shader_ir.h