[llvm] [AArch64] Improve getPartialReductionCost for fixed-width VFs (PR #126538)

[Sam Tebbs via llvm-commits]

SamTebbs33 wrote:

> Hi @SamTebbs33, after rebasing the patch I noticed some failing tests due to the tests assuming fixed-width partial reductions were legal for 64-bit accumulators. I've tried to fix the epilogue test by changing the accumulator to 32 bits instead, but I'm not sure if it's still testing the thing you want to.

I believe that test was checking cases when the epilogue VF is smaller than the main body VF and it's still doing that so I think it still looks good. Thank you for fixing this!

https://github.com/llvm/llvm-project/pull/126538