New optimization to break up statements in void contexts.

	PR 6644
	* elaborate.cxx (dead_stmtexpr_remover::visit_block): Flatten nested
	block statements into a single block.
	(dead_stmtexpr_remover::visit_if_statement): Remove the possibility
	of if_statements with a null thenblock.  When an if lacks both then
	and else, either remove it completely or reduce it to a simple
	statment evaluating the condition.  With an else and no then, invert
	the condition and else becomes then.
	(void_statement_reducer): New optimization visitor that breaks
	statements in void context into smaller pieces, to expose more
	optimization opportunities.
	(semantic_pass_opt5, semantic_pass_opt6): Bump opt5 to opt6, and
	create a new opt5 that runs through void_statement_reducer.

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