combine: Don't do make_more_copies for dest PC (PR87720)

Jumps are written in RTL as moves to PC.  But the latter has no mode,
so we shouldn't try to use it.  Since the optimization this routine
does does not really help for jumps at all, let's just skip it.


	PR rtl-optimization/87720
	* combine.c (make_more_copies): Skip if the dest is pc_rtx.

From-SVN: r265474