Fix reuse of non-dominating synthesized value in subregion exit

We were adding all generated values in non-affine subregions to be used
for the subregions generated exit block. The thought was that only
values that are dominating the original exit block can be used there.
But it is possible for synthesizable values to be expanded in any
block. If the same values is also used for implicit writes, it would
try to reuse already synthesized values even if not dominating the exit
block.

The fix is to only add values to the list of values usable in the exit
block only if it is dominating the exit block. This fixes
llvm.org/PR25412.

llvm-svn: 252301