[llvm] [VPlan] Remove VPBlendRecipe and replace with select VPInstructions (PR #150369)

[Luke Lau via llvm-commits]

lukel97 wrote:

Rebased now that #150368 and #150357 landed.

@fhahn I'm not strongly opinionated about this so happy to just leave this PR if you want to keep VPBlendRecipe. But I thought it would be worthwhile sharing the code, since it doesn't look like much depends on keeping blends abstract early on. 

https://github.com/llvm/llvm-project/pull/150369