Revert "Revert "[flang][hlfir] Fixed assignment/finalization order for...

Revert "Revert "[flang][hlfir] Fixed assignment/finalization order for user-defined assignments. (#66736)""

This reverts commit 775754e3.

Relanding with removing part of the LIT test. There seems to be operations
ordering indeterminism that is unrelated to my change. I will address this
issue separately.