[MLIR][IntegerRangeAnalysis] Avoid crash reached when loop bound is uninitialized (#74832)

If the loop bound is not initialized, the analysis crashed, as it only checked for nullity. Also checking for initialization fixes the issue.

Signed-off-by: Victor Perez <victor.perez@codeplay.com>
Co-authored-by: Tsang, Whitney <whitney.tsang@intel.com>