Port load/store op translation to LLVM IR dialect lowering

Implement the lowering of memref load and store standard operations into the
LLVM IR dialect.  This largely follows the existing mechanism in
MLIR-to-LLVM-IR translation for the sake of compatibility.  A memref value is
transformed into a memref descriptor value which holds the pointer to the
underlying data buffer and the dynamic memref sizes.  The data buffer is
contiguous.  Accesses to multidimensional memrefs are linearized in row-major
form.  In linear address computation, statically known sizes are used as
constants while dynamic sizes are extracted from the memref descriptor.

PiperOrigin-RevId: 233043846