shader: Fix structured control flow on KIL instructions

This could potentially leave unvisited blocks, leading to illegal phi
nodes.

src/shader_recompiler/frontend/ir/ir_emitter.cpp

@@ -83,6 +83,7 @@ void IREmitter::SelectionMerge(Block* merge_block) {
 }
 
 void IREmitter::Return() {
+    block->SetReturn();
     Inst(Opcode::Return);
 }