+
+{- from John Dias's patch 2008/10/16:
+ The linear-scan allocator sometimes allocates a block
+ before allocating one of its predecessors, which could lead to
+ inconsistent allocations. Make it so a block is only allocated
+ if a predecessor has set the "incoming" assignments for the block, or
+ if it's the procedure's entry block.
+
+ BL 2009/02: Careful. If the assignment for a block doesn't get set for
+ some reason then this function will loop. We should probably do some
+ more sanity checking to guard against this eventuality.
+-}