[mlir][Transform] Fix crash in SequenceOp::getEffects when body region is empty (#185063)
When walking operations post-order and erasing blocks, the inner body
block of a nested transform.sequence can be erased while the outer op is
still alive. If printAsOperand is called on the outer block at that
point, it triggers verification, which calls SequenceOp::getEffects ->
getPotentialTopLevelEffects -> getBodyBlock() -> Region::front() on an
empty region, causing an assertion failure in ilist_iterator
('\!NodePtr->isKnownSentinel()').
Fix by checking that the body region is non-empty before passing its
front block to detail::getPotentialTopLevelEffects in the
PossibleTopLevelTransformOpTrait.
Fixes #60213
Assisted-by: Claude Code
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