middle-end/93519 - avoid folding stmts in obviously unreachable code

middle-end/93519 - avoid folding stmts in obviously unreachable code

The inliner folds stmts delayed, the following arranges things so
to not fold stmts that are obviously not reachable to avoid warnings
from those code regions.

2020-02-07  Richard Biener  <rguenther@suse.de>

	PR middle-end/93519
	* tree-inline.c (fold_marked_statements): Do a PRE walk,
	skipping unreachable regions.
	(optimize_inline_calls): Skip folding stmts when we didn't
	inline.

	* gcc.dg/Wrestrict-21.c: New testcase.