shader_recompiler, video_core: Resolve clang errors

Silences the following warnings-turned-errors:
-Wsign-conversion
-Wunused-private-field
-Wbraced-scalar-init
-Wunused-variable

And some other errors

src/shader_recompiler/backend/glasm/emit_context.h

@@ -59,7 +59,7 @@ public:
     }
 
     std::string code;
-    RegAlloc reg_alloc{*this};
+    RegAlloc reg_alloc{};
     const Info& info;
     const Profile& profile;
     const RuntimeInfo& runtime_info;