[Impeller] Update guidance on shader optimization to include recent findings and tools.

[chinmaygarde]

I was reading the still excellent guidance on shader optimizations in the docs. Since the doc was written, we have wired up shader static analysis on CI using ARM Mali tools. Also, we have learned more about the effects of uniform computation and varying loads. We should update the linked guidance based on our current profiling workflows and knowledge.

References: go/impeller-slow-fragment-shader and go/impeller-push-constants

[chinmaygarde]

cc @jonahwilliams @bdero @gaaclarke