Concretize gimple_call_set_fn

Concretize gimple_call_set_fn

This corresponds to:
  [PATCH 82/89] Concretize gimple_call_set_fntype
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01187.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> This is fine once prerequisites have gone in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00840.html

gcc/
	* gimple.h (gimple_call_set_fn): Require a gimple_call.

	* trans-mem.c (dump_tm_memopt_transform): Likewise.
	(tm_memopt_transform_blocks): Add checked casts to gimple_call in
	suites guarded by is_tm_simple_{load|store}, which enforce that
	the statement must be a GIMPLE_CALL; use this when invoking
	dump_tm_memopt_transform.