Initial support for SLP tree throttling. Trims non-profitable subtrees, trying to maximize perf gains. Does not support trees with gathered loads yet, since they are not quite trees, but graphs. Analysis should be added later. Reviewers: RKSimon Pull Request: https://github.com/llvm/llvm-project/pull/162018 Recommit after revert in 6ec2ec4826b51d7d809fe08b36883a78d7dc0b98 with a fix