shader: isolate SWAPPC work from ordinary shader paths

src/graphics/shader/recompiler/frontend/decode/ShaderDecoder.h

@@ -818,6 +818,7 @@ struct Instruction {
 struct Program {
 	std::span<const uint32_t> code;
 	std::vector<Instruction>  instructions;
+	bool                     has_swap_pc = false;
 };
 
 // Code spans are trusted to contain complete instructions, valid branch targets, and 32-bit PCs.