shader: Use shared_ptr to store nodes and move initialization to file

Instead of having a vector of unique_ptr stored in a vector and
returning star pointers to this, use shared_ptr. While changing
initialization code, move it to a separate file when possible.

This is a first step to allow code analysis and node generation beyond
the ShaderIR class.

src/video_core/shader/decode/xmad.cpp

@@ -5,6 +5,7 @@
 #include "common/assert.h"
 #include "common/common_types.h"
 #include "video_core/engines/shader_bytecode.h"
+#include "video_core/shader/node_helper.h"
 #include "video_core/shader/shader_ir.h"
 
 namespace VideoCommon::Shader {