Improve common reduction vs builtin code generation in loop distribution

loop distribution currently cannot handle the situation when the
last partition is a builtin but there's a common reduction in all
partitions (like the final IV value).  The following lifts this
restriction by making the last non-builtin partition provide the
definitions for the loop-closed PHI nodes.  Since we have heuristics
in place to avoid code generating builtins last writing a testcase
is difficult (but I ran into a case with other pending patches that
made the heuristic ineffective).  What's remaining is the inability
to preserve common reductions when all partitions could be builtins
(in some cases final value replacement could come to the rescue here).

	* tree-loop-distribution.cc (copy_loop_before): Add
	the ability to replace the original LC PHI defs.
	(generate_loops_for_partition): Pass through a flag
	whether to redirect original LC PHI defs.
	(generate_code_for_partition): Likewise.
	(loop_distribution::distribute_loop): Compute the partition
	that should provide the LC PHI defs for common reductions
	and pass that down.

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