[NFC][LoopVectorize] Restructure simple early exit tests (#112721)

The previous simple_early_exit.ll was growing too large and difficult to
manage. Instead I've decided to refactor the tests by splitting out into
notional groups:

1. single_early_exit.ll: loops with a single uncountable exit that do
not have live-outs from the loop.
2. single_early_exit_live_outs.ll: loops with a single uncountable exit
with live-outs.
3. multi_early_exit.ll: loops with multiple early exits, i.e. a mixture
of countable and uncountable exits, but with no live-outs from the loop.
4. multi_early_exit_live_outs.ll: as above, but with live-outs.
5. single_early_exit_unsafe_ptrs.ll: loops with a single uncountable
exit, but with pointers that are not unconditionally dereferenceable.
6. unsupported_early_exit.ll: loops with uncountable exits that we
cannot yet vectorise.
7. early_exit_legality.ll: tests the debug output from
LoopVectorizationLegality to make sure we handle different scenarios
correctly.

Only the last test now requires asserts. Over time some of these tests
should start vectorising as more support is added.

I also tried to rename the multi early exit tests to make it clear there
what mixture of countable and uncountable exits are present.

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