[llvm] [LoopVectorize] Add support for vectorisation of more early exit loops (PR #88385)

[David Sherwood via llvm-commits]

david-arm wrote:

I've rebased this patch on top of PR #107004, but it also includes a few other PRs that are in flight: PR #109300, PR #108563 and PR #108231. If/when those patches I can rebase this PR and it should shrink in size considerably.

This patch still needs work, for example:
1. VPLiveOut::fixPhi needs updating to use the incoming block parameter instead of the current hack.
2. addUsersInEarlyExitBlock needs updating to use the same model as addUsersInExitBlock, i.e. collect users separately and build up a vector of them. The vectoriser and vplan has changed considerably since April when I first put the patch up. :) Ideally, I'd find a way to unify the code between the normal and early exits.
3. createInitialVPlan needs tidying up a bit - do we really need to pass in the exiting and exit blocks? and do we need to keep a record of the original early exit?

https://github.com/llvm/llvm-project/pull/88385