shader/memory_util: Deduplicate code

Deduplicate code shared between vk_pipeline_cache and gl_shader_cache as
well as shader decoder code.

While we are at it, fix a bug in gl_shader_cache where compute shaders
had an start offset of a stage shader.

src/video_core/CMakeLists.txt

@@ -124,6 +124,8 @@ add_library(video_core STATIC
     shader/decode.cpp
     shader/expr.cpp
     shader/expr.h
+    shader/memory_util.cpp
+    shader/memory_util.h
     shader/node_helper.cpp
     shader/node_helper.h
     shader/node.h