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

src/backend/executor/nodeModifyTable.c

@@ -832,9 +832,11 @@ ExecInsert(ModifyTableState *mtstate,
 			 *
 			 * We loop back here if we find a conflict below, either during
 			 * the pre-check, or when we re-check after inserting the tuple
-			 * speculatively.
+			 * speculatively.  Better allow interrupts in case some bug makes
+			 * this an infinite loop.
 			 */
 	vlock:
+			CHECK_FOR_INTERRUPTS();
 			specConflict = false;
 			if (!ExecCheckIndexConstraints(resultRelInfo, slot, estate,
 										   &conflictTid, arbiterIndexes))