[llvm] [AArch64] Fix SVE scalar fcopysign lowering without neon. (PR #129787)

[via llvm-commits]

llvmbot wrote:

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Failed to cherry-pick: d4ab3df320f9eebf11cc5fb600a0919f93678abe

https://github.com/llvm/llvm-project/actions/runs/13684753931

Please manually backport the fix and push it to your github fork.  Once this is done, please create a [pull request](https://github.com/llvm/llvm-project/compare)

https://github.com/llvm/llvm-project/pull/129787