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

The recently introduced iterative constant import
(https://reviews.llvm.org/D137559) fails for programs that
subsequently import constant expressions with duplicate
subexpressions. The reason is a broken duplicate check
in getConstantsToConvert. The revision fixes the bug and
adds a test case that imports two constant expressions
with duplicates.

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D139918