[llvm] [DAG] Prevent combination on small negative value for vscale (PR #88108)

[via llvm-commits]

vfdff wrote:

> Conflicting review comments but if you're happy taking the isel approach then I'd rather maintain the current canonicalisation.

Thanks your suggestion.
Yes, It is expected to improve instruction selection,  but break the  canonicalisation.


https://github.com/llvm/llvm-project/pull/88108