[flang][hlfir] Expand array hlfir.assign's.

Expand hlfir.assign with in-memory array RHS and LHS into
a loop nest with element-by-element assignments.
For small arrays this may result in further loop nest unrolling
enabling more value propagation and redundancy elimination.

Note the change in flang/test/HLFIR/opt-bufferization.fir:
the hlfir.assign inside hlfir.elemental gets expanded by the new
pattern.

Depends on D159151

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D159246