tree-ssa-forwprop.c: Use gassign

tree-ssa-forwprop.c: Use gassign

gcc/ChangeLog.gimple-classes:
	* tree-ssa-forwprop.c (get_prop_dest_stmt): Capture result of
	gimple_assign_ssa_name_copy_p as new local "use_assign", using in
	place of "use_stmt" for typesafety.
	(remove_prop_source_from_use): Add checked cast.
	(rhs_to_tree): Strengthen param "stmt" from gimple to gassign *.
	(forward_propagate_into_comparison): Strengthen local "stmt" from
	gimple to gassign *.
	(forward_propagate_into_cond): Likewise.
	(combine_cond_exprs): Likewise.  Replace local bool "changed" with
	gimple "changed_stmt", adding assignments to "changed_stmt" in
	place of those to "changed".  Replace is_gimple_assign with a
	dyn_cast, using it to strenghten local "def_stmt" from gimple to
	gassign *.
	(tidy_after_forward_propagate_addr): Add checked cast.
	(forward_propagate_addr_expr_1): Strengthen local "use_stmt" from
	gimple to gassign *.
	(forward_propagate_addr_expr): Introduce local "use_assign", using
	it in place of "use_stmt" for typesafety.
	(forward_propagate_comparison): Strengthen local "stmt" from
	gimple to gassign *.  Replace is_gimple_assign with a dyn_cast,
	using it to introduce local "use_assign", using it in place of
	"use_stmt" for typesafety.
	(simplify_conversion_from_bitmask): Strengthen local "stmt" from
	gimple to gassign *.  Replace is_gimple_assign with a dyn_cast,
	using it to introduce local "rhs_def_assign", using it in place of
	"rhs_def_stmt" for typesafety.
	(simplify_not_neg_expr): Strengthen local "stmt" from gimple to
	gassign *.  Replace is_gimple_assign with a dyn_cast,
	using it to strengthen local "rhs_def_stmt" from gimple to
	gassign *.
	(simplify_gimple_switch): Add checked cast.
	(constant_pointer_difference): Replace is_gimple_assign with a
	dyn_cast, using it to strengthen local "stmt" from gimple to
	gassign *.
	(simplify_builtin_call): Capture result of gimple_assign_single_p
	as new local "assign_stmt1", using it in place of "stmt1" for
	typesafety.
	(truth_valued_ssa_name): Add checked cast.
	(lookup_logical_inverted_value): Replace is_gimple_assign with a
	dyn_cast, using it to strengthen local "def" from gimple to
	gassign *.
	(simplify_bitwise_binary_boolean): Likewise for local
	"op0_def_stmt".  Strengthen local "stmt" from gimple to gassign *.
	(simplify_bitwise_binary):  Strengthen local "stmt" from gimple to
	gassign *.
	(simplify_rotate): Likewise.
	(associate_pointerplus_align): Likewise, also for local "def_stmt"
	via various is_gimple_assign becoming dyn_casts.
	(associate_pointerplus_diff): Likewise, also for local "minus".
	(combine_conversions): Likewise.
	(simplify_vce): Likewise.
	(simplify_bitfield_ref): Likewise.
	(simplify_permutation): Likewise.
	(simplify_vector_constructor): Likewise.
	(simplify_mult): Likewise.
	(fold_all_stmts): Capture result of gimple_assign_single_p as new
	local "assign_stmt", using it in place of "stmt" for typesafety.
	(pass_forwprop::execute): Within case GIMPLE_ASSIGN add new
	local "assign_stmt" and use it in place of "stmt" for typesafety.