[flang][hlfir] deref pointers before lowering assignment to hlfir.assign

[flang][hlfir] deref pointers before lowering assignment to hlfir.assign

There is little point not to dereference pointers LHS and RHS before
before emitting an hlfir.assign when lowering an assignment.
This pushes complexity and descriptor read side effects that are better
expressed in a load before the assignment.

Differential Revision: https://reviews.llvm.org/D143372