tree-optimization/95855 - Add checks to avoid spoiling if-conversion if

tree-optimization/95855 - Add checks to avoid spoiling if-conversion if

Add some checks in pass_splits_paths, so that pass_split_paths can
recognize the missed if-conversion opportunity and do not duplicate the
corresponding block.

2020-06-30 Yang Yang <yangyang305@huawei.com>

gcc/ChangeLog:

	PR tree-optimization/95855
	* gimple-ssa-split-paths.c (is_feasible_trace): Add extra
	checks to recognize a missed if-conversion opportunity when
	judging whether to duplicate a block.

gcc/testsuite/ChangeLog

	PR tree-optimization/95855
	* gcc.dg/tree-ssa/split-path-12.c: New testcase.