Fix builtin vs non-builtin partition merge in loop distribution

Fix builtin vs non-builtin partition merge in loop distribution

When r7-6373-g40b6bff965d004 fixed a costing issue it failed to
make the logic symmetric which means that we now fuse
normal vs. builtin when the cost model says so but we don't fuse
builtin vs. normal.  The following fixes that, also allowing
the cost model to decide to fuse two builtin partitions as otherwise
an intermediate non-builtin can result in a partial merge as well.

	* tree-loop-distribution.cc (loop_distribution::distribute_loop):
	When computing cost-based merging do not disregard builtin
	classified partitions in some cases.

	* gcc.dg/tree-ssa/ldist-24.c: XFAIL.
	* gcc.dg/tree-ssa/ldist-36.c: Adjust expected outcome.