Shortening #-of Vector Lanes

[itzmeanjan]

Previously I represented Rescue Prime hash state using vector with 16 lanes, where each element was 64 -bit unsigned integer ( i.e. sycl::ulong16 ), motivated by discussion here rust-lang/portable-simd#215 , I decided to rewrite rescue prime hash routines, while representing hash state using sycl::ulong4[3], which is exactly 12 -field elements wide; not wasting any space like I was doing before.

See how performance of Rescue Prime Hash and Merklization improves after aforementioned change is benchmarked on Nvidia Tesla V100

And on Intel CPU perform improvements due to vector lane shortening are visible here

Rescue Prime Merge

Platform	# -of Work Items	Op/ sec
Intel CPU	4096 x 4096	1.41329e+06
Nvidia GPU	4096 x 4096	2.02718e+07

Merklization using Rescue Prime Merge

Platform	# -of Work Items	Total Time
Intel CPU	8388608	5974.54 ms
Nvidia GPU	8388608	431.217 ms