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

[Nicholas Guy via llvm-commits]

NickGuy-Arm wrote:

> > 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?

No runtime perf-impact has been observed, but costing a loop with e.g. VF=2 should be the same regardless of whether maxBandwidth is enabled, whereas currently such a VF may be discarded with maxBandwidth enabled.

As for compile-time impact, we have observed some small improvements (up to 2.5%) on some workloads when MaxBandwidth is enabled with this change vs without.

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