[flang][hlfir] Expand hlfir.assign's with scalar RHS.

Expanding hlfir.assign's with scalar RHS late in MLIR optimization
pipeline allows LLVM to recognize most of them as simple memset loops.
This is especially important for small size LHS arrays, because
the assign loop nest may be completely unrolled enabling more value
propagation.

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D159151