tree-ssa-sink.c: Use gassign

gcc/ChangeLog.gimple-classes:
	* tree-ssa-sink.c (statement_sink_location): Rename param "stmt"
	to "gs", reintroducing "stmt" as a local gassign * via a dyn_cast
	for typesafety.
tree-ssa-strlen.c: Use gassign

gcc/ChangeLog.gimple-classes:
	* tree-ssa-strlen.c (find_equal_ptrs): Replace is_gimple_assign
	with a dyn_cast, strengthening local "stmt" from gimple to
	gassign *.
	(adjust_last_stmt): Likewise, introducing local "last_assign" and
	using it in place of last.stmt, and strengthening local "def_stmt"
	from gimple to gassign *.
	(handle_builtin_memcpy): Replace is_gimple_assign with a dyn_cast,
	strengthening local "def_stmt" from gimple to gassign *.
	(handle_pointer_plus): Strengthen local "stmt" from gimple to
	gassign *. Add checked cast.
	(handle_char_store): Strengthen local "stmt" from gimple to
	gassign *.
	(strlen_optimize_stmt): Replace is_gimple_assign with dyn_cast,
	introducing local gassign * "assign_stmt", using it in place of
	"stmt" for typesafety.
tree-ssa-structalias.c: Use gassign

gcc/ChangeLog.gimple-classes:
	* tree-ssa-structalias.c (find_func_aliases): Replace
	is_gimple_assign with a dyn_cast, introducing local gassign *
	"t_assign", using it in place of "t" for typesafety.
	(find_func_clobbers): Add checked cast.
tree-ssa-tail-merge.c: Use gassign

gcc/ChangeLog.gimple-classes:
	* tree-ssa-tail-merge.c (same_succ_hash): Add checked cast.
	(gimple_equal_p): Add checked casts.
tree-ssa-ter.c: Use gassign

gcc/ChangeLog.gimple-classes:
	* tree-ssa-ter.c (find_replaceable_in_bb): Replace
	is_gimple_assign with a dyn_cast, introducing local "def_assign"
	and using it in place of "def_stmt" for typesafety.  Add a checked
	cast.
tree-ssa-threadedge.c: Use gassign

gcc/ChangeLog.gimple-classes:
	* tree-ssa-threadedge.c (lhs_of_dominating_assert): Capture result
	of gimple_assign_single_p as new local gassign * "use_assign",
	using it in place of "use_stmt" for typesafety.
	(fold_assignment_stmt): Strengthen param "stmt" from gimple to
	gassign *.
	(record_temporary_equivalences_from_stmts_at_dest): Replace check
	against GIMPLE_ASSIGN with a dyn_cast, introducing local gassign *
	"assign_stmt", using it in place of "stmt" for typesafety.  Later,
	use it to capture the result of gimple_assign_single_p, and use it
	in place of "stmt" for typesafety.  Add checked cast.