[flang][hlfir] Fixed associate-op codegen for optimized HLFIR.

This effectively reverts D154715.

The issue appears as the dialect conversion error because we try to
erase an op that has already been erased. See the added LIT test case
with HLFIR that may appear as a result of CSE.
The `adaptor.getSource()` is an operation producing a tuple,
which does not have users, so `allOtherUsesAreSafeForAssociate`
just looks at the empty list of users. So we get completely wrong
answers from it. This causes problems with the following
`eraseAllUsesInDestroys` that tries to remove the `DestroyOp` twice
during both `hflir.associate` processing.

But we also cannot use `associate.getSource()` *efficiently*, because
the original users may still hang around: one example is the original body
of hlfir.elemental (see D154715), another example is other already converted
AssociateOp's that are pending removal in the rewriter
(that is why we have a temporary created for each hlfir.associate
in the newly added LIT case).

This patch just fixes the correctness issue. I think we have to separate
the buffer reuse analysis from the conversion itself.

I also tried to address the issues with the cloned bodies of `hlfir.elemental`,
but this should not matter since D155778: if `hlfir.associate` is inside
`hlfir.elemental`, it will end up inside a do-loop body region, so the early
exit added in D155778 will prevent the buffer reuse.

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D158471