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

[Paul Walker via llvm-commits]

paulwalker-arm 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?

Whilst true the precedent has been set and thus the easiest fix is effectively repeat the crime.  I'd rather leave such refactoring as a separate activity if there's value in doing so.

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