findSelectionMerge took the nearest common post-dominator of a selection's
targets, computed across the loop's back edge. When one arm only continues
(reaching that block in a later iteration) and the other target is shared
with an enclosing selection, the shared merge was split off the header, the
post-dominator stayed where it was, and splitting repeated until the budget
threw.
Inside a loop, an arm that reaches the post-dominator only past the continue
or merge block now leaves the selection by continuing, and the merge is the
other arm's target; after one split that is the header's own synthetic merge.
agc_structurizer_tests builds the smallest such graph; it hit the budget
before.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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