[VRP] Use alloc-pool and obstack for value_range and vr->equiv allocations

[kugan <kugan dot vivekanandarajah at linaro dot org>]

Hi Richard,

As discussed in IPA-VRP discussion, this patch makes tree-vrp 
allocations use alloc-pool and obstack for value_range and vr->equiv 
respectively. Other allocations are rare and left as it is.

Bootstrapped and regression tested on x86-64-linux with no new 
regressions. Is this OK for trunk.

Thanks,
Kugan


gcc/ChangeLog:

2016-07-20  Kugan Vivekanandarajah  <kuganv@linaro.org>

	* tree-vrp.c (set_value_range): Use vrp_equiv_obstack with
	BITMAP_ALLOC.
	(add_equivalence): Likewise.
	(get_value_range): Allocate value range with vrp_value_range_pool.
	(vrp_initialize): Initialize vrp_equiv_obstack for equiv allocation.
	(vrp_finalize): Relase vrp_equiv_obstack and vrp_value_range_pool.

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