[flang][hlfir] Preserve hlfir.assign in the bufferize-hlfir pass

hlfir.assign must be rewritten in the bufferize pass since its operands,
that can be expressions, may have been updated. This is just
an operand update rewrite. The previous code was replacing the
operation, but it was dropping all hlfir.assign on the floor doing
so. This broke allocatable assignment semantics that use attributes.

Update the operands in place instead to preserve the attributes, if any.

Differential Revision: https://reviews.llvm.org/D148310