combine: Do another check before splitting a parallel (PR86771)

combine: Do another check before splitting a parallel (PR86771)

When combine splits a resulting parallel into its two SETs, it has to
place one at i2, and the other stays at i3.  This does not work if the
destination of the SET that will be placed at i2 is modified between
i2 and i3.  This patch fixes it.

	* combine.c (try_combine): Do not allow splitting a resulting PARALLEL
	of two SETs into those two SETs, one to be placed at i2, if that SETs
	destination is modified between i2 and i3.

From-SVN: r263776