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

[via llvm-commits]

vfdff wrote:

Can the refactoring approach be like this? 
 > Make sure the  const immediate is a positive constant value in the VScale and delete the pattern that matches a negative value from the tablegen.

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