[flang][hlfir] Apply MemAlloc effect to hlfir.elemental explicitly.

Related to https://github.com/llvm/llvm-project/issues/64866.
This patch effectively disables CSE for identical hlfir.elemental
operations, because it causes hlfir.destroy to be applied twice
to the same temporary. Moreover, I think MemAlloc is correct
for hlfir.elemental, in general.

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D158565