[llvm] [AArch64] Enable maximising scalable vector bandwidth (PR #166748)

[Sam Tebbs via llvm-commits]

SamTebbs33 wrote:

> I wasn't aware of this TTI hook. But I agree it has a very promising name, and why wouldn't we want to set this? Did you run some perf numbers with this change? Can you share what improved?

Sorry for the delay! V2 has shown some small improvements in perlbench (0.74%), cam4 (1.94%) and fotonik3d (2.77%). V1 shows improvements in xalancbmk (1.58%), wrf (0.74%) and fotonik3d (0.53%), but regresses in others so is disabled.

https://github.com/llvm/llvm-project/pull/166748