Performance regression when baking LODs with the GPULM

This was detected by our performance tests:

https://observer.prd.ds.unity3d.com/stonks/Unity.GI/variations/editor/pivots/editor/changesets/6a05ce51ff3c 

The commit range to look into is between `d832162ee654ad4fc1c34ab4dfe55986670f5b06` (good) and `ba913132852ff450c8508fac2f47e7a454c8b7c6` (bad). 

How to reproduce:

On a Windows machine with a modern GPU – 

Using the project under unity/unity: 

1. Open the test project Tests/PerformanceTests/GI
2. Run the UnityEditor.GI.Tests.Performance.LevelsOfDetail.ManyInstancesWithLODGroupWithLOD("0",ProgressiveGPU) test. 
3. Note the Bake Time Total and Bake Performance under the performance test report window.

You will run into compilation errors in the bisection commit range (which is why we haven't been able to do it automatically). When you do, apply the following changes to the project: [Internal link] . You can just copy the entire file from here [Internal link] and replace the existing Tests/PerformanceTests.cs.

Note that I can't provide expected / actual numbers as they depend on your hardware. The regression is that Bake Time Total is about 4-5 times larger and Bake Performance is equally lower than before. See the Observer link above.