Schedule reduction partition in the last.

Schedule reduction partition in the last.

If reduction partition's SCC is broken by runtime alias checks, force
a negative post order to it so that it will be scheduled in the last.

2020-07-09  Bin Cheng  <bin.cheng@linux.alibaba.com>

gcc/
	PR tree-optimization/95804
	* tree-loop-distribution.c (break_alias_scc_partitions): Force
	negative post order to reduction partition.

gcc/testsuite/
	PR tree-optimization/95804
	* gcc.dg/tree-ssa/pr95804.c: New test.