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/CMakeLists.txt

@@ -102,6 +102,8 @@ add_library(video_core STATIC
     shader/decode/xmad.cpp
     shader/decode/other.cpp
     shader/decode.cpp
+    shader/node_helper.cpp
+    shader/node_helper.h
     shader/shader_ir.cpp
     shader/shader_ir.h
     shader/track.cpp