[mlir][DCA] Fix visiting call ops when run at function scopes

When dead-code analysis is run at the scope of a function, call ops to
other functions at the same level were being marked as unreachable,
since the analysis optimistically assumes the call op to have no known
predecessors and that all predecessors are known, but the callee would
never get visited.

This patch fixes the bug by checking if a referenced function is above
the top-level op of the analysis, and is thus considered an external
callable.

Fixes #56830

Reviewed By: zero9178

Differential Revision: https://reviews.llvm.org/D130829