Clean up and virtualize the on-entry cache interface.

Clean up and virtualize the on-entry cache interface.

Cleanup/Virtualize the ssa_block_range class, and implement the current
vector approach as a derived class.
Allow memory allocation from the irange allocator obstack for easy freeing.

	* gimple-range-cache.cc (ssa_block_ranges): Virtualize.
	(sbr_vector): Renamed from ssa_block_cache.
	(sbr_vector::sbr_vector): Allocate from obstack abd initialize.
	(ssa_block_ranges::~ssa_block_ranges): Remove.
	(sbr_vector::set_bb_range): Use varying and undefined cached values.
	(ssa_block_ranges::set_bb_varying): Remove.
	(sbr_vector::get_bb_range): Adjust assert.
	(sbr_vector::bb_range_p): Adjust assert.
	(~block_range_cache): No freeing loop required.
	(block_range_cache::get_block_ranges): Remove.
	(block_range_cache::set_bb_range): Inline get_block_ranges.
	(block_range_cache::set_bb_varying): Remove.
	* gimple-range-cache.h (set_bb_varying): Remove prototype.
	* value-range.h (irange_allocator::get_memory): New.