[mlir] Extended BufferPlacement to support more sophisticated scenarios in which

allocations cannot be moved freely and can remain in divergent control flow.

The current BufferPlacement pass does not support allocation nodes that carry
additional dependencies (like in the case of dynamic shaped types). These
allocations can often not be moved freely and in turn might remain in divergent
control-flow branches. This requires a different strategy with respect to block
arguments and aliases. This CL adds additinal functionality to support
allocation nodes in divergent control flow while avoiding memory leaks.

Differential Revision: https://reviews.llvm.org/D79850