[llvm] [DominanceFrontier] Support post-dominators (PR #179336)

[Andrei Elovikov via llvm-commits]

https://github.com/eas commented:

> To check I understand correctly, in https://github.com/llvm/llvm-project/pull/173265 we explicitly check if the header block is post dominated by a given block, but with this patch we can generalise it to any block in the frontier right? I.e. we wouldn't need to explicitly specify the header block.

No, generalization of #173265 is to use IDom instead of header for mask re-use. The frontier part would build on top of that, something like https://github.com/eas/llvm-project/commit/bd2d5c290ce148517a1d36ff12f9f51602f293e9#diff-8e3cb833c9fc7eb3cfd25bda5197da648aa1cf5f47440067969c3c9553694849, look at VPlanTests/predicator.ll for the meaningful test change.

https://github.com/llvm/llvm-project/pull/179336