Avoid block access() check for blocks not using the fallback formatter in LayoutBuilderBuildSubscriber

Problem/Motivation

LayoutBuilderBuildSubscriber::onBuildRender calls $block->access() on every FieldBlock before checking whether the block actually uses the field_fallback_formatter formatter. On layouts with many field blocks this runs an access check per block for nothing; only blocks configured with the fallback formatter need it.

Depending on the number of blocks in the layout and the cost of the access check, this can save anywhere from a little to a lot of per-request rendering time.

Steps to reproduce

Render an entity via Layout Builder with several FieldBlock components; observe $block->access() being called for blocks not using the fallback formatter.

Proposed resolution

Move the formatter-type check (and the mandatory-config check) above the $block->access() call.

Remaining tasks

?

User interface changes

None

API changes

None

Data model changes

None