MIR: Do not require END_BLOCK to always exist

Once upon a time, along with START_BLOCK and END_BLOCK in the castle of important blocks also lived
a RESUME_BLOCK (or was it UNWIND_BLOCK? Either works, I don’t remember anymore). This trinity of
important blocks were required to always exist from the birth to death of the MIR-land they
belonged to.

Some time later, it was discovered that RESUME_BLOCK was just a lazy goon enjoying comfortable life
in the light of fame of the other two. Needless to say, once found out, the RESUME_BLOCK was
quickly slain and disposed of.

Now, the all-seeing eye of ours discovers that END_BLOCK is actually the more evil and better
disguised twin of the slain RESUME_BLOCK. Thus END_BLOCK gets slain and quickly disposed
of. Glory to the START_BLOCK, one and only lord of the important blocks’ castle!

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Basically, all this does, is removing restriction for END_BLOCK to exist past the first invocation
of RemoveDeadBlocks pass. This way for functions whose CFG does not reach the `END_BLOCK` end up
not containing the block.

As far as the implementation goes, I’m not entirely satisfied with the `BasicBlock::end_block`, I
had hoped to make `new` a `const fn` and then just have a `const END_BLOCK` private to mir::build,
but it turns out that constant functions don’t yet support conditionals nor a way to assert.

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