Concretize gimple_cond_{lhs|rhs}_ptr

Concretize gimple_cond_{lhs|rhs}_ptr

This corresponds to:
  [PATCH 76/89] Concretize gimple_cond_{lhs|rhs}_ptr
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01170.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/msg00851.html

gcc/
	* gimple.h (gimple_cond_lhs_ptr): Require a const_gimple_cond
	rather than just a const_gimple_cond.
	(gimple_cond_rhs_ptr): Likewise.

	* gimplify-me.c (gimple_regimplify_operands): Add a checked cast
	to gimple_cond within "case GIMPLE_COND".
	* omp-low.c (lower_omp_1): Likewise.

	* omp-low.c (expand_omp_simd): Introduce a new local cond_stmt
	to express that the conditional is indeed a gimple_cond.

	* tree-ssa-loop-ivopts.c (extract_cond_operands): Add a checked
	cast to gimple_cond within a region where the code is known to
	be GIMPLE_COND.