Migrate skip checks to --EXTENSIONS--, p3

For rationale, see #6787

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

Fix phi use chain management when renaming variable

If there is a previous use of the new variable in the phi, we need
to NULL out the use chain of the new source we're adding.

Fix phi use chain management when renaming variable

If there is a previous use of the new variable in the phi, we need
to NULL out the use chain of the new source we're adding.

Test case is reduced from an assertion failure in the Symfony Demo.

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