tree-optimization/101151 - fix irreducible region check for sinking

tree-optimization/101151 - fix irreducible region check for sinking

The check whether two blocks are in the same irreducible region
and thus post-dominance checks being unreliable was incomplete
since an irreducible region can contain reducible sub-regions but
if one block is in the irreducible part and one not the check
still doesn't work as expected.

2021-06-22  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101151
	* tree-ssa-sink.c (statement_sink_location): Expand irreducible
	region check.

	* gcc.dg/torture/pr101151.c: New testcase.