LLVM IR lowering: support vector_type_cast

Introduce support for lowering vector_type_cast to LLVM IR.  It consists in
creating a new MemRef descriptor with the base pointer with the type that
corresponds to the lowered element type of the target memref.  Since
`vector_type_cast` does not support dynamic shapes in the target type, no
dynamic size conversion is necessary.

This commit goes in the opposite direction of what is expected of LLVM IR
lowering: it should not be aware of all the other dialects.  Instead, we should
have separate definitions for conversions in a global lowering framework.
However, this requires LLVM dialect to be implemented, which is currently
blocked by the absence of user-defined types.  Implement the lowering anyway to
unblock end-to-end vectorization experiments.
PiperOrigin-RevId: 225887368