Use gimple_call in some places within tree-ssa-dom.c

Use gimple_call in some places within tree-ssa-dom.c

This corresponds to:
  [PATCH 87/89] Use gimple_call in some places within tree-ssa-dom.c
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01173.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK when prereqs go in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00856.html

gcc/
	* tree-ssa-dom.c (struct hashable_expr): Strengthen field
	call.fn_from from gimple to gimple_call.
	(initialize_hash_element): Replace check against GIMPLE_CALL
	with dyn_cast<gimple_call> and update gimple_call_ uses to use
	new gimple_call local, along with fn_from initializer.
	(iterative_hash_hashable_expr): Strengthen type of local "fn_from"
	from gimple to gimple_call.
	(print_expr_hash_elt): Likewise.