[llvm] [AArch64] Fix throughout of 64-bit SVE gather loads (PR #168572)

[Cullen Rhodes via llvm-commits]

c-rhodes wrote:

> > why isn't it possible to get the correct throughput with the existing resources?
> 
> Because the pipeline used by gather loads is unit L, which has 3 resources. This makes it so the throughput is a result of a division by 3.

Doesn't that imply a bug somewhere? I don't understand why is the throughput 4/5 if it's not possible to get that with the resources as documented in the SWOG. Looking at the other neoverse cores they all use some of the vector pipes for these gathers, are we sure the SWOG is correct?

Also "Non temporal gather load, vector + scalar 32-bit element size" is 4 micro-ops whereas 64-bit element size is 2 micro-ops, that doesnt make sense.

https://github.com/llvm/llvm-project/pull/168572