[patch] Use a smaller, dynamic worklist in compute_global_livein

[Steven Bosscher <stevenb dot gcc at gmail dot com>]

Hello,

In the test case for PR54146, compute_global_livein allocates/frees a
worklist for >400,000 basic blocks on each invocation. And it's called
a lot, for rewrite_into_loop_closed_ssa. But the maximum number of
basic blocks ever on the work list was only ~6500. So the work list
can be much smaller most of the time.

Bootstrapped&tested on x86_64-unknown-linux-gnu. OK for trunk?

Ciao!
Steven

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