This patch adds an inchash hasher for wide_int-based types.
It means that hash_tree no longer hashes TREE_INT_CST_EXT_NUNITS,
but that was redundant with hashing the type.

2017-10-22  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* inchash.h (inchash::hash::add_wide_int): New function.
	* lto-streamer-out.c (hash_tree): Use it.

From-SVN: r253988