Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

Extensions migrated in part 3:
* ftp
* gmp
* iconv
* opcache
* shmop

Always print numeric opline numbers

Improve type inference for COALESCE

Place a pi node on the non-null edge to remove a spurious
undef/null type.

Additionally, adjust the profitability heuristic to be more
ac

Improve type inference for COALESCE

Place a pi node on the non-null edge to remove a spurious
undef/null type.

Additionally, adjust the profitability heuristic to be more
accurate if the "other predecessor" writes to the variable.
Ideally this should not just consider the direct predecessors,
but it's sufficient for this case.

This partially addresses bug #79353 by removing the discrepancy
between ?? and ??=.

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