shader: keep fragment layer values under lane masks

Co-authored-by: shadowbeat070 <70843316+shadowbeat070@users.noreply.github.com>

src/graphics/shader/recompiler/ir/passes/ConstantPropagation.h

@@ -4,6 +4,9 @@
 
 namespace Libs::Graphics::ShaderRecompiler::IR {
 
+struct Program;
+
 void ConstantPropagationPass(const BlockList& blocks);
+void EliminateMaskedValues(Program& program);
 
 } // namespace Libs::Graphics::ShaderRecompiler::IR