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

[David Green via llvm-commits]

davemgreen wrote:

Hi - sorry I didn't get to this yesterday. Tablegen patterns that produce multiple results are often a bad idea so my vote would probably be for less of them if we can! Perhaps there could be a isLegalVScaleImm that we combine/legalize towards?

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