See https://github.com/llvm/llvm-project/pull/147168 for more info.

let constructor is legacy (do not use in tree!) since the tableGen
backend emits most of the glue logic to build a pass.

Note: The following constructor has been retired:

```cpp
std::unique_ptr<Pass> createExpandReallocPass(bool emitDeallocs = true);
```
    
To update your codebase, replace it with the new options-based API:
    
```cpp
memref::ExpandReallocPassOptions expandAllocPassOptions{
          /*emitDeallocs=*/false};
pm.addPass(memref::createExpandReallocPass(expandAllocPassOptions));
```

This change cleans up call sites. Next step is to mark the member
functions deprecated.

See https://mlir.llvm.org/deprecation and
https://discourse.llvm.org/t/preferred-casting-style-going-forward.

There are two motivations for this change:
1. It considerably simplifies adding support for the realloc operation to the
   new buffer deallocation pass by lowering the realloc such that no
   deallocation operation is inserted and the deallocation pass itself can
   insert that dealloc
2. The lowering is expressed on a higher level and thus easier to understand,
   and the lowerings of the memref operations it is composed of don't have to
   be duplicated in the MemRefToLLVM lowering (also see discussion in
   https://reviews.llvm.org/D133424)

Reviewed By: springerm

Differential Revision: https://reviews.llvm.org/D159430