[llvm] [LoongArch] Add demanded bits support for [X]VMSKLTZ (PR #143528)

[Lu Weining via llvm-commits]

================
@@ -8192,3 +8210,56 @@ unsigned LoongArchTargetLowering::getNumRegistersForCallingConv(
 
   return TargetLowering::getNumRegistersForCallingConv(Context, CC, VT);
 }
+
+bool LoongArchTargetLowering::SimplifyDemandedBitsForTargetNode(
+    SDValue Op, const APInt &OriginalDemandedBits,
+    const APInt &OriginalDemandedElts, KnownBits &Known, TargetLoweringOpt &TLO,
+    unsigned Depth) const {
+  EVT VT = Op.getValueType();
+  unsigned BitWidth = OriginalDemandedBits.getBitWidth();
+  unsigned Opc = Op.getOpcode();
+  switch (Opc) {
+  case LoongArchISD::VMSKLTZ:
+  case LoongArchISD::XVMSKLTZ: {
+    SDValue Src = Op.getOperand(0);
+    MVT SrcVT = Src.getSimpleValueType();
+    unsigned SrcBits = SrcVT.getScalarSizeInBits();
+    unsigned NumElts = SrcVT.getVectorNumElements();
+
+    // If we don't need the sign bits at all just return zero.
+    if (OriginalDemandedBits.countr_zero() >= NumElts)
+      return TLO.CombineTo(Op, TLO.DAG.getConstant(0, SDLoc(Op), VT));
+
+    // Only demand the vector elements of the sign bits we need.
+    APInt KnownUndef, KnownZero;
+    APInt DemandedElts = OriginalDemandedBits.zextOrTrunc(NumElts);
+    if (SimplifyDemandedVectorElts(Src, DemandedElts, KnownUndef, KnownZero,
+                                   TLO, Depth + 1))
+      return true;
+
+    Known.Zero = KnownZero.zext(BitWidth);
+    Known.Zero.setHighBits(BitWidth - NumElts);
+
+    // [X]VMSKLTZ only uses the MSB from each vector element.
+    KnownBits KnownSrc;
+    APInt DemandedSrcBits = APInt::getSignMask(SrcBits);
+    if (SimplifyDemandedBits(Src, DemandedSrcBits, DemandedElts, KnownSrc, TLO,
+                             Depth + 1))
+      return true;
+
+    if (KnownSrc.One[SrcBits - 1])
+      Known.One.setLowBits(NumElts);
+    else if (KnownSrc.Zero[SrcBits - 1])
+      Known.Zero.setLowBits(NumElts);
+
+    // Attempt to avoid multi-use os if we don't need anything from it.
----------------
SixWeining wrote:

```suggestion
    // Attempt to avoid multi-use ops if we don't need anything from it.
```

https://github.com/llvm/llvm-project/pull/143528