pgsql: Add CHECK_FOR_INTERRUPTS in ExecInsert's speculative insertion l

Add CHECK_FOR_INTERRUPTS in ExecInsert's speculative insertion loop.

Ordinarily the functions called in this loop ought to have plenty
of CFIs themselves; but we've now seen a case where no such CFI is
reached, making the loop uninterruptible.  Even though that's from
a recently-introduced bug, it seems prudent to install a CFI at
the loop level in all branches.

Per discussion of bug #17558 from Andrew Kesper (an actual fix for
that bug will follow).

Discussion: https://postgr.es/m/17558-3f6599ffcf52fd4a@postgresql.org

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/efba7a63ffbe59790b88e3af459dfd81c742b90f

Modified Files
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src/backend/executor/nodeModifyTable.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)