gl_shader_disk_cache: Skip stored shader variants instead of asserting

Instead of asserting on already stored shader variants, silently skip them.
This shouldn't be happening but when a shader is invalidated and it is
not stored in the shader cache, this assert would hit and save that
shader anyways when the asserts are disabled.

src/video_core/renderer_opengl/gl_shader_disk_cache.cpp

@@ -475,7 +475,10 @@ void ShaderDiskCacheOpenGL::SaveUsage(const ShaderDiskCacheUsage& usage) {
     ASSERT_MSG(it != transferable.end(), "Saving shader usage without storing raw previously");
 
     auto& usages{it->second};
-    ASSERT(usages.find(usage) == usages.end());
+    if (usages.find(usage) != usages.end()) {
+        // Skip this variant since the shader is already stored.
+        return;
+    }
     usages.insert(usage);
 
     FileUtil::IOFile file = AppendTransferableFile();