[llvm] [LV] Add test showing lack of gather/scatter can prevent if-convert (PR #195042)

[Gaëtan Bossu via llvm-commits]

gbossu wrote:

I'd be happy to split the patch further, I also think that having a central place to query whether a target supports masked accesses makes sense in isolation. 

But there's still one problem that remains. I'd like this new helper to replace all calls to `TTI.isLegalMaskedLoad`/`TTI.isLegalMaskedStore`, otherwise the patch makes little sense IMO. This means changing:
 - isScalarWithPredication
 - interleavedAccessCanBeWidened

We can either do that by changing `isConsecutivePointer` to support strides (which isn't great I agree) or by hoisting the access pattern legality checks out of `isLegalMaskedLoadOrStore` as I proposed in https://github.com/llvm/llvm-project/pull/194579#discussion_r3167349594.

https://github.com/llvm/llvm-project/pull/195042