inchash::hash::add_wide_int operated directly on the raw encoding
of the wide_int, including any redundant upper bits.  The problem
with that is that the upper bits are only defined for some wide-int
storage types (including wide_int itself).  wi::to_wide(tree) instead
returns a value that is extended according to the signedness of the
type (so that wi::to_widest can use the same encoding) while rtxes
have the awkward special case of BI, which can be zero-extended
rather than sign-extended.

In the PR, we computed a hash for a "normal" sign-extended wide_int
while the existing entries hashed wi::to_wide(tree).  This gives
different results for unsigned types that have the top bit set.

The patch fixes that by hashing the canonical sign-extended form even
if the raw encoding happens to be different.

2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* wide-int.h (generic_wide_int::sext_elt): New function.
	* inchash.h (hash::add_wide_int): Use it instead of elt.

From-SVN: r273881