[llvm] [LLVM][VPlan] Keep all VPBlend masks until VPlan transformation. (PR #104015)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

Thanks @ayalz. I tried to follow the existing implementation where possible to minimise the chances of breaking stuff but I'll create some followup PRs to try out switching the operand order along with the other suggestions.

https://github.com/llvm/llvm-project/pull/104015