Revamp irange_allocator to handle vranges.

This patch revamps the range allocator to handle generic vrange's.
I've cleaned it up somehow to make it obvious the various things you
can allocate with it.  I've also moved away from overloads into
distinct names when appropriate.

The various entry points are now:

  // Allocate a range of TYPE.
  vrange *alloc_vrange (tree type);
  // Allocate a memory block of BYTES.
  void *alloc (unsigned bytes);
  // Return a clone of SRC.
  template <typename T> T *clone (const T &src);

It is now possible to allocate a clone of an irange, or any future
range types:

      irange *i = allocator.clone <irange> (some_irange);
      frange *f = allocator.clone <frange> (some_frange);

You can actually do so without the <>, but I find it clearer to
specify the vrange type.

So with it you can allocate a specific range type, or vrange, or a
block of memory.

I have rewritten the C style casts to C++ casts, since casts tend to
be hints of problematic designs.  With the C++ casts you can at least
grep for them easier.  Speak of which, the next patch, which converts
ranger to vrange, will further clean this space by removing some
unnecessary casts.

Tested on x86-64 Linux and ppc64le Linux.

	* gimple-range-cache.cc (sbr_vector::sbr_vector): Adjust for
	vrange allocator.
	(sbr_vector::grow): Same.
	(sbr_vector::set_bb_range): Same.
	(sbr_sparse_bitmap::sbr_sparse_bitmap): Same.
	(sbr_sparse_bitmap::set_bb_range): Same.
	(block_range_cache::~block_range_cache): Same.
	(block_range_cache::set_bb_range): Same.
	(ssa_global_cache::ssa_global_cache): Same.
	(ssa_global_cache::~ssa_global_cache): Same.
	(ssa_global_cache::set_global_range): Same.
	* gimple-range-cache.h (block_range_cache): Same.
	(ssa_global_cache): Same.
	* gimple-range-edge.cc
	(gimple_outgoing_range::calc_switch_ranges): Same.
	* gimple-range-edge.h (gimple_outgoing_range): Same.
	* gimple-range-infer.cc (infer_range_manager::get_nonzero):
	Same.
	(infer_range_manager::add_range): Same.
	* gimple-range-infer.h (class infer_range_manager): Same.
	* value-range.h (class irange_allocator): Rename to...
	(class vrange_allocator): ...this.
	(irange_allocator::irange_allocator): New.
	(vrange_allocator::vrange_allocator): New.
	(irange_allocator::~irange_allocator): New.
	(vrange_allocator::~vrange_allocator): New.
	(irange_allocator::get_memory): Rename to...
	(vrange_allocator::alloc): ...this.
	(vrange_allocator::alloc_vrange): Rename from...
	(irange_allocator::allocate): ...this.
	(vrange_allocator::alloc_irange): New.