[llvm] [PowerPC] Fix vector_shuffle combines when inputs are scalar_to_vector of differing types. (PR #80784)

[zhijian lin via llvm-commits]

================
@@ -15371,33 +15381,52 @@ SDValue PPCTargetLowering::combineVectorShuffle(ShuffleVectorSDNode *SVN,
     // the value into element zero. Since scalar size of LHS and RHS may differ
     // after isScalarToVec, this should be checked using their own sizes.
     if (SToVLHS) {
-      if (!IsLittleEndian && SToVLHS.getValueType().getScalarSizeInBits() >= 64)
+      int LHSScalarSize = SToVLHS.getValueType().getScalarSizeInBits();
+      if (!IsLittleEndian && LHSScalarSize >= 64)
         return Res;
       // Set up the values for the shuffle vector fixup.
-      LHSMaxIdx = NumEltsOut / NumEltsIn;
+      LHSNumValidElts =
+          LHSScalarSize / LHS.getValueType().getScalarSizeInBits();
+      // The last element that comes from the LHS. For example:
+      // (shuff (s_to_v i32), (bitcast (s_to_v i64), v4i32), ...)
+      // The last element that comes from the LHS is actually 0, not 3
+      // because elements 1 and higher of a scalar_to_vector are undefined.
+      LHSLastElt = LHSScalarSize / (ShuffleEltWidth + 1);
       SToVLHS = getSToVPermuted(SToVLHS, DAG, Subtarget);
       if (SToVLHS.getValueType() != LHS.getValueType())
         SToVLHS = DAG.getBitcast(LHS.getValueType(), SToVLHS);
       LHS = SToVLHS;
     }
     if (SToVRHS) {
-      if (!IsLittleEndian && SToVRHS.getValueType().getScalarSizeInBits() >= 64)
+      int RHSScalarSize = SToVRHS.getValueType().getScalarSizeInBits();
+      if (!IsLittleEndian && RHSScalarSize >= 64)
         return Res;
-      RHSMinIdx = NumEltsOut;
-      RHSMaxIdx = NumEltsOut / NumEltsIn + RHSMinIdx;
+      RHSNumValidElts =
+          RHSScalarSize / RHS.getValueType().getScalarSizeInBits();
+      // The last element that comes from the RHS. For example:
+      // (shuff (s_to_v i32), (bitcast (s_to_v i64), v4i32), ...)
+      // The last element that comes from the RHS is actually 5, not 7
+      // because elements 1 and higher of a scalar_to_vector are undefined.
+      // It is also not 4 because the original scalar_to_vector is wider and
+      // actually contains two i32 elements.
+      RHSLastElt = RHSScalarSize / (ShuffleEltWidth + 1) + RHSFirstElt;
----------------
diggerlin wrote:

most of code 
```
if (SToVLHS) {
  ... 
}
```

as same with 
```
if (SToVRHS) {
... 
}
```

can you add a help function or lambda function to simply the code ?

https://github.com/llvm/llvm-project/pull/80784