Compute LCP savings for `render-blocking-resources`

https://github.com/GoogleChrome/lighthouse/pull/15238#discussion_r1258902571

`render-blocking-resources` computes savings using the FCP graph. In theory this savings estimate should be fine to use for LCP as well since render blocking resources are always loaded before FCP (i.e. none affect LCP and not FCP).

I do wonder how far this assumption goes, and if it's worth it to use the same calculation. Perhaps making a resource non-render blocking doesn't completely eliminate it's LCP impact.

Start by reviewing the render-blocking-resources audit and the linked pull request discussion. Determine whether its FCP-based savings calculation can be applied to LCP, and validate whether removing render-blocking status can still affect LCP. Done means the LCP savings behavior and its supporting evidence are clearly established.

Written by the indexing model from the issue text.