This rewrites most of make_more_copies, in the process fixing a few PRs
and some other bugs, and working around a few target problems.  Certain
notes turn out to actually change the meaning of the RTL, so we cannot
drop them; and i386 takes subregs of hard regs.


	PR rtl-optimization/87701
	PR rtl-optimization/87780
	* combine.c (make_more_copies): Rewrite.

From-SVN: r265582