16.2.9: Clarify DU/DA within a block-statement-group

JLS2 16.2.8 included a clause relating DU/DA in one block-statement-group to DU/DA in the preceding block-statement-group. That clause was removed in JLS3 16.2.9, but should have been replaced with:

V is [un]assigned before a block-statement other than the first of any block-statement-group iff V is [un]assigned after the preceding block-statement in the block-statement-group.