[llvm] [AArch64] Combine getActiveLaneMask with vector_extract (PR #81139)

[Momchil Velikov via llvm-commits]

momchil-velikov wrote:

> Sorry for the delay. To me it looks like you're doing type legalisation as a DAG combine, which while a similar crime is being committed for fixed length types I don't think you should repeat because it will prevent any legitimate DAG combines. Custom type legalisation really belongs in `ReplaceNodeResults`.

I didn't think I was doing type legalisation as I wasn't changing any types. Anyway, certainly I meant for this transform to be a plain 
 simple DAG combine on a type-legalised DAG, so I moved it to a later phase (by pivoting on `aarch64_sve_whilelo` instead of `get_active_lane_mask`).

https://github.com/llvm/llvm-project/pull/81139