[llvm] [LV] Create block in mask up-front if needed. (PR #76635)

[via llvm-commits]

https://github.com/ayalz approved this pull request.

This looks good to me, thanks! Adding some minor nits, and thoughts:

One alternative may be to continue generating masks on-demand, but place them at top of block, e.g., by having createBlockInMask() bump its Builder (temporarily) to first non-phi position in block. Another alternative is to have (subsequent) users of a mask move it above them, if/when needed.
Note that even if one generates all masks eagerly, as first non-phi's in block, it's possible for other non-phi's to be inserted above them. In general, predication should be done as a one-shot VPlan-to-VPlan transform (according to roadmap), including all (needed) masks and their predicated users.

https://github.com/llvm/llvm-project/pull/76635