+;;; This function used to extend an existing filler instead of creating
+;;; a new one when the previous segment annotation was a filler. Now
+;;; this is only done if the previous filler is immediately adjacent
+;;; to the new one in the segment, too. To see why this restriction is
+;;; necessary, consider a jump followed by an alignment made of
+;;; multi-byte NOPs when both are shrunk: The shortened alignment is
+;;; reemitted at its original _start_ position but the joined filler
+;;; would extend over this position and instead leave a subsequence of
+;;; the segment up to the alignment's original _end_ position visible.