Hoist if block selectors

[Rich-Harris]

It occurred to me that the get_block functions, which determine which block to render in a series of if...elseif...else blocks, can always be hoisted. Should save a microscopic amount of work when if blocks are inside each blocks, with no downside that I can think of.

I also renamed get_block to select_block_type (and current_block to current_block_type) since I think that's marginally more descriptive of what's happening.