[mlir][llvm] Fix bug in constant import from LLVM IR.

The revision addresses a bug during constant expression traversal
when importing LLVM IR. A constant expression may have cyclic
dependencies, for example, when a constant is initialized with its
address. This revision extends the constant expression traversal
to detect cyclic dependencies and adds a test to verify this
case is handled properly.

Reviewed By: Dinistro

Differential Revision: https://reviews.llvm.org/D143152