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

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

It's likely due to the `AddedComplexity = 5 ` making them look better than the `UseScalarIncVL` patterns that use the same value of complexity.  Do the new patterns [1] need any added complexity? as they really only need to be used in preference to the general `vscale` pattern that emits `MADDXrrr`.  I'd start with removing it and see if that resolves the issue.  If not then reducing its value should.

[1] I think you'll need to have all the `! UseScalarIncVL` patterns use complexity consistently so that it doesn't use the `SUBXrs XZR` patterns when using `SUBXrs GPR64:$op` is preferred.

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