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hash: use logical right shift in ihash

This aligns ihash with the SplittableRandom/SplitMix64 function,
which is defined using logical shifts.

[0] https://xorshift.di.unimi.it/splitmix64.c

--- a/fns.h

+++ b/fns.h

@@ -30,7 +30,7 @@

 void	reclaimblk(Bptr);

 ushort	blkfill(Blk*);

 uvlong	blkhash(Blk*);

-u32int	ihash(vlong);

+u32int	ihash(uvlong);

 void	finalize(Blk*);

 Tree*	newsnap(Tree*);

 char*	freesnap(Tree*, Tree*);

--- a/hash.c

+++ b/hash.c

@@ -109,7 +109,7 @@

 }

 u32int

-ihash(vlong x)

+ihash(uvlong x)

 {

     x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9ULL;

     x = (x ^ (x >> 27)) * 0x94d049bb133111ebULL;