Fix DomTree preservation for generated subregions.

Fix DomTree preservation for generated subregions.

The generated dedicated subregion exit block was assumed to have the same
dominance relation as the original exit block. This is incorrect if the exit
block receives other edges than only from the subregion, which results in that
e.g. the subregion's entry block does not dominate the exit block.

llvm-svn: 261865