[llvm] [VPlan] Simplify select with constant condition (PR #150357)

[Luke Lau via llvm-commits]

lukel97 wrote:

> The InstSimplifyFolder is used independently of the VPBuilder, by matching live-ins in simplifyRecipe. I think the broadcast_splat operand in the changed tests aren't live-ins, and that's the reason the folder fails. Any opinions on extending tryToFoldLiveIns to handle the case when _some_ operands are live-ins? We're already doing a pretty extensive match there, and true/false in the select cond should be constant live-ins.

Oh sorry you're right, I tried this out and it's because we don't handle VPWidenSelectRecipes with the InstSimplifyFolder. I'll rework this PR to support it

https://github.com/llvm/llvm-project/pull/150357