[MLIR][IR] Rename `Block::hasTerminator` to `mightHaveTerminator` (#66870)

This `Block` member function introduced in
87d77d3c

 may be misleading to users as
the last operation in the block might have not been registered, so there
would be no way to ensure that is a terminator.

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Signed-off-by: Victor Perez <victor.perez@codeplay.com>