incorrect hashing of large floats

[simonbyrne]

For Float64s on the interval (2^63, 2^64), the hashes don't match those of the corresponding Uints (or BigFloats):

julia> x = 0x1.8p63
1.3835058055282164e19

julia> u = convert(Uint,x)
0xc000000000000000

julia> hash(x)
0x9815276104e1427d

julia> hash(u)
0xf9d22a93486bf9f9

julia> hash(big(x))
0xf9d22a93486bf9f9

The problem is that on this line, fptosi always returns -typemin(Int64) for all values >= 2^63 (this is the same problem as in #7517).

[StefanKarpinski]

Oh, man, this stuff is so tricky to change. I'll have to think on it for a bit.