[llvm] [AArch64][SVE] Expose flags result of predicate-as-counter whiles (NFC) (PR #202976)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

Mostly, creating this PR for opinions on this lowering as it's a different approach from the SVE1 whiles, but allows for similar optimizations (but at an earlier stage in the pipeline). The goal is to enable DAG combines like:

```c++
// Fold extract(pext(whilelo_pred_counter, 0), 0) to
// cset(whilelo_pred_counter, first_active).
if (sd_match(N,
              m_ExtractElt(m_IntrinsicWOChain<Intrinsic::aarch64_sve_pext>(
                              m_SpecificOpc(AArch64ISD::WHILELO_PRED_COUNTER),
                              m_Zero()),
                          m_Zero()))) {
  SDValue WhileLO = N0->getOperand(1);
  SDValue Flags = SDValue(WhileLO.getNode(), 1);
  return getSETCC(AArch64CC::CondCode::FIRST_ACTIVE, Flags, SDLoc(N), DAG);
}
```




https://github.com/llvm/llvm-project/pull/202976