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Fix FOLD rule for BUFHDR append.

Reported by XmiliaH.

(cherry-picked from commit bc1bdbf62)

`bufput_append()` may fold `BUFHDR RESET` + `BUFPUT` IRs to `BUFHDR
APPEND` even if the right operand (`BUFSTR`) is the PHI. If it's not the
last IR in the `BUFSTR` chain, this may lead to an incorrect resulting
value in the buffer, which contains a longer string since `APPEND` is
used instead of `RESET`.

This patch adds the corresponding check inside the fold rule.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#9145

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