Concentrate memref descriptor manipulation logic in one place

Concentrate memref descriptor manipulation logic in one place

Memref descriptor is becoming increasingly complex. Memrefs are manipulated by
multiple standard instructions, each of which has a non-trivial lowering to the
LLVM dialect. This leads to verbose code that manipulates the descriptors
exposing the internals of insert/extractelement opreations. Implement a wrapper
class that contains a memref descriptor and provides semantically named methods
that build the primitive IR operations instead.

PiperOrigin-RevId: 280371225