Introduce gimple_bind and use it for accessors.

Introduce gimple_bind and use it for accessors.

This corresponds to:
  [PATCH 03/89] Introduce gimple_bind and use it for accessors.
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01190.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> This is fine, with the same requested changes as #2; specifically
> using an explicit cast
> rather than hiding the conversion in a method. Once those changes are
> in place, it's good for 4.9.1.
in https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01485.html

This updates all of the gimple_bind_* accessors in gimple.h from taking a
plain gimple to taking a gimple_bind (or const_gimple_bind), with the
checking happening at the point of cast.

Various other types are strengthened from gimple to gimple_bind, and from
plain vec<gimple> to vec<gimple_bind>.

gcc/
	* coretypes.h (gimple_bind): New typedef.
	(const_gimple_bind): New typedef.

	* gdbhooks.py (build_pretty_printer): Add gimple_bind
	and its variants, using the gimple printer.

	* gimple-pretty-print.c (dump_gimple_bind): Update type-signature to
	require a gimple_bind rather than just a gimple.

	* gimple.c (gimple_build_bind): Return a gimple_bind rather than
	just a gimple.
	* gimple.h (gimple_build_bind): Likewise.

	* gimple.h (gimple_seq_first_stmt_as_a_bind): New.

	* gimple.h (gimple_bind_vars): Update type-signature to
	require a gimple_bind rather than just a gimple, removing
	as_a and hence run-time check.
	(gimple_bind_set_vars): Likewise.
	(gimple_bind_append_vars): Likewise.
	(gimple_bind_body_ptr): Likewise.
	(gimple_bind_body): Likewise.
	(gimple_bind_set_body): Likewise.
	(gimple_bind_add_stmt): Likewise.
	(gimple_bind_add_seq): Likewise.
	(gimple_bind_block): Likewise.
	(gimple_bind_set_block): Likewise.
	* gimplify.c (gimple_push_bind_expr): Likewise.
	(gimple_current_bind_expr): Likewise.
	* tree-inline.c (copy_gimple_bind): Likewise.

	* gimplify.h (gimple_current_bind_expr): Return a gimple_bind
	rather than a plain gimple.
	(gimplify_body): Likewise.
	(gimple_bind_expr_stack): Return a vec<gimple_bind> rather than
	a vec<gimple>.

	* gimplify.c (struct gimplify_ctx): Strengthen field
	"bind_expr_stack" from vec<gimple> to vec<gimple_bind>.
	(gimple_bind_expr_stack): Likewise for type of returned value.

	* gimplify.c (gimplify_body): Strengthen various types from gimple
	to gimple_bind, including the return type.

	* gimplify.c (declare_vars): Introduce "gs" as a generic gimple,
	so that local "scope" can be of type gimple_bind once we've reached
	the region where it must be of code GIMPLE_BIND.

	* gimple-low.c (lower_gimple_bind): Add checked cast to
	gimple_bind, since both callers (lower_function_body and
	lower_stmt) have checked the code for us.

	* gimple.c (gimple_copy): Add checked cast to gimple_bind in
	region guarded by check for code GIMPLE_BIND.
	* gimple-low.c (gimple_stmt_may_fallthru): Likewise.
	* gimple-pretty-print.c (pp_gimple_stmt_1): Likewise.
	* gimple-walk.c (walk_gimple_stmt): Likewise.
	* omp-low.c (scan_omp_1_stmt): Likewise.
	(lower_omp_1): Likewise.
	(lower_omp_for): Likewise.
	* tree-cfg.c (verify_gimple_in_seq_2): Likewise.
	(do_warn_unused_result): Likewise.
	* tree-inline.c (remap_gimple_stmt): Likewise.
	(estimate_num_insns): Likewise.
	* tree-nested.c (convert_nonlocal_reference_stmt): Likewise.

	* gimplify.c (gimplify_bind_expr): Update local(s) to be a
	gimple_bind rather than just a gimple.
	(gimplify_function_tree): Likewise.
	* omp-low.c (lower_omp_sections): Likewise.
	(lower_omp_single): Likewise.
	(lower_omp_master): Likewise.
	(lower_omp_taskgroup): Likewise.
	(lower_omp_ordered): Likewise.
	(lower_omp_critical): Likewise.
	(lower_omp_taskreg): Likewise.
	(lower_omp_teams): Likewise.
	* omp-low.c (lower_omp_for): Likewise; use
	gimple_seq_first_stmt_as_a_bind to encapsulate the checked cast.
	(lower_omp_target): Likewise.
	* tree-nested.c (finalize_nesting_tree_1): Likewise.

	* gimple.c (empty_stmt_p): Add dyn_cast to a gimple_bind.
	* tree-inline.c (replace_locals_stmt): Add dyn_cast to gimple_bind.

gcc/c-family/
	* c-gimplify.c (add_block_to_enclosing): Strengthen local "stack"
	from being just a vec<gimple> to a vec<gimple_bind>.

gcc/java/
	* java-gimplify.c (java_gimplify_block): Update local to be a
	gimple_bind rather than just a gimple.

Conflicts:
	gcc/omp-low.c
	gcc/tree-nested.c