Concretize gimple_call_set_tail and gimple_call_tail_p

Concretize gimple_call_set_tail and gimple_call_tail_p

This corresponds to:
  [PATCH 83/89] Concretize gimple_call_set_tail and gimple_call_tail_p
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01172.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK once prereqs go in.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00859.html

gcc/
	* gimple.h (gimple_call_set_tail): Require a gimple_call.
	(gimple_call_tail_p): Likewise.

	* cfgexpand.c (expand_gimple_tailcall): Likewise.
	(expand_gimple_basic_block): Convert calls to is_gimple_call to a
	dyn_cast, introducing a new "call_stmt" local.

	* trans-mem.c (expand_block_edges): Likewise, for comparison against
	GIMPLE_CALL.

	* tree-inline.c (remap_gimple_stmt): Add checked casts to
	gimple_call in region guarded by is_gimple_call.

	* tree-tailcall.c (optimize_tail_call): Add checked cast to gimple_call
	for t->call_gsi.