[mlir][llvm] Improve LLVM IR constant import.

Improve the constant import to handle zeroinitializer as well as
additional float types such as quad floats. The logic got restructured
to avoid creating intermediate dense element attributes when
constructing multi-dimensional arrays. Additionally, we also leverage
the fact that we do not need to iterate all elements of splat constants.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D150274