Eliminate some of more expensive gcc_asserts

[Jan Hubicka]

> Hi,
> this patch change most of asserts that collect over 500 samples in my
> oprofile to gcc_checking_assert.  I tried to double check that they
> are not terribly important.
> In bitmap.c they are all bitmap internal checks, I kept those checking
> that things are used correctly.
> 
> Bootstrapped/regtested x86_64-linux, OK?
> 
> 	* bitmap.c (bitmap_and, bitmap_and_into, bitmap_and_compl,
> 	bitmap_and_compl_into, bitmap_compl_and_into, bitmap_ior,
> 	bitmap_ior_into, bitmap_xor, bitmap_xor_into,
> 	bitmap_ior_and_compl, bitmap_ior_and_compl): Turn internal datastructure
> 	checks into checking asserts.
> 	* rtlanal.c (find_reg_note): Use gcc_checking_assert.
> 	* tree-ssa-sccvn.c (VN_INFO): Likewise.
> 	* df-scan.c (df_reorganize_refs_by_reg_by_reg, df_install_ref,
> 	df_ref_create_structure): Likewise.
> 	* alloc-pool.c (create_alloc_pool, empty_alloc_pool, pool_alloc,
> 	pool_free): Use gcc_checking_assert.
> 	* alias.c (get_alias_set): Likewise.
> 	* var-tracking.c (variable_htab_free, shared_hash_copy,
> 	canonicalize_values_mark, variable_merge_over_cur): Likewise.
> 	* lto-streamer.c (bp_unpack_value): Likewise.

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