Fix narrow bounds checking in generic gathers

[lpetersen-pixel]

I noticed that generic gather did not work for certain values;
for a 128-element source, 1...128', an int8_tindex of 127 should return 128, but currently returns 0 as we callpartial_gather_from` which clamps the upper bound to the index types maximum and tests index < bound.

solution: simply retain the full range size and compare safely; I would assume this adds a branch, but havent checked.

[danieltowner]

@lpetersen-pixel Thanks for your repot and fix. Coincidentally I found something very similar recently and have a patch sitting on my machine waiting to be applied. I'll merge/compare with yours when I get back from cppcon.

[lpetersen-pixel]

@lpetersen-pixel Thanks for your repot and fix. Coincidentally I found something very similar recently and have a patch sitting on my machine waiting to be applied. I'll merge/compare with yours when I get back from cppcon.

no worries, thanks for the great talk :)

[danieltowner]

@lpetersen-pixel I took your basic fix and created a more comprehensive PR which also added verification and support for AVX-512 (#36). Thanks for your help!