[llvm] [LoopVectorizer] Prune VFs based on plan register pressure (PR #132190)

[Sam Tebbs via llvm-commits]

SamTebbs33 wrote:

I've raised a PR to revert to previous behaviour, where the register pressure was only considered if vector bandwidth was being maximised. I've had a look at the assembly output and the vplan for the reproducer, and noticed that the output has a lot of `vmla`s, so I think a long term solution would be to hide the register usage of the plan's multiplies so that it still gets vectorised when register usage is considered.

https://github.com/llvm/llvm-project/pull/132190