Add CHECK_FOR_INTERRUPTS in ExecInsert's speculative insertion loop.

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