Support loop structures with FHE operations

[j2kun]

We'd like to support loop structures in HEIR that contain FHE operations. Much of the auxiliary compiler work is done, as loop support for most lowerings and code generation was required for the layout system to compile ISL-generated packing and unpacking loops.

The remaining work is to implement the core of the HALO compiler paper (see #289)

Sub issues filed related to this:

• https://github.com/google/heir/issues/1181
• https://github.com/google/heir/issues/1569
• https://github.com/google/heir/issues/289