[llvm] [VPlan] Compute blend masks from minimum set of edge masks (PR #184838)

[Luke Lau via llvm-commits]

lukel97 wrote:

Mentioning here that this PR is equivalent to #186478 if you only consider post dominance on the subgraph of the CFG that leads to the block containing the phi. So if we only considered the subgraph we wouldn't need to iterate the postdom frontier.

I explored making a custom GraphTraits that only traversed the subgraph but it ended up being more trouble than it's worth. It would also mean recomputing a new DomFrontierBase for every block we need to insert blends, and DomFrontierBase computes the frontier for every block in the graph so it's likely much slower than the current approach. 

https://github.com/llvm/llvm-project/pull/184838