Concretize get_loop_exit_condition et al to working on gimple_cond

Concretize get_loop_exit_condition et al to working on gimple_cond

This corresponds to:
  [PATCH 18/89] Concretize get_loop_exit_condition et al to working on gimple_cond
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01215.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK when prerequisites have gone in.
> Actually that's true for #17 & #18 as well.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00626.html

gcc/
	* tree-scalar-evolution.h (get_loop_exit_condition): Return a
	gimple_cond.
	* tree-scalar-evolution.c (get_loop_exit_condition): Likewise, also
	concretizing local "res" from gimple to gimple_cond.
	* tree-vect-loop-manip.c (slpeel_make_loop_iterate_ntimes): Convert
	locals from gimple to gimple_cond.
	(slpeel_can_duplicate_loop_p): Likewise.
	* tree-vect-loop.c (vect_get_loop_niters): Return a gimple_cond.
	(vect_analyze_loop_form): Convert local from gimple to gimple_cond.