gimple-low.c (struct lower_data): Add cannot_fallthru field.

	* gimple-low.c (struct lower_data): Add cannot_fallthru field.
	(lower_stmt) <GIMPLE_BIND>: Add comment.
	<GIMPLE_COND, GIMPLE_GOTO, GIMPLE_SWITCH>: Set cannot_fallthru to true
	and return.
	<GIMPLE_RETURN>: Remove the statement if cannot_fallthru is set.
	Otherwise lower it and set cannot_fallthru to true.
	<GIMPLE_TRY>: Update cannot_fallthru for GIMPLE_TRY_FINALLY and return.
	<GIMPLE_CATCH, GIMPLE_EH_FILTER>; Set cannot_fallthru to false.
	<GIMPLE_CALL>: Set cannot_fallthru to false for BUILT_IN_SETJMP and
	to true for a noreturn call.  Do not remove statements.
	<GIMPLE_OMP_PARALLEL, GIMPLE_OMP_TASK>: Set cannot_fallthru to false.
	Set cannot_fallthru to false on function exit.
	(gimple_stmt_may_fallthru) <GIMPLE_SWITCH>: Really return false.
	<GIMPLE_ASSIGN>: Remove.

From-SVN: r152984