[llvm] [LoopVectorizer] Only check register pressure for VFs that have been enabled via maxBandwidth (PR #149056)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> Theoretically this allows for higher VFs to be considered than would otherwise be deemed "safe" (from a regpressure perspective), but more concretely this reduces the amount of work done at compile-time when maxBandwidth is enabled.

Is there any expected perf-impact from the change? If so, is there any perf data you could share?

https://github.com/llvm/llvm-project/pull/149056