[clang] [Headers][X86] VectorExprEvaluator::VisitCallExpr / InterpretBuiltin - allow PALIGNR byte shift intrinsics to be used in constexpr (PR #162005)

[Simon Pilgrim via cfe-commits]

================
@@ -13097,6 +13106,33 @@ bool VectorExprEvaluator::VisitCallExpr(const CallExpr *E) {
 
     return Success(APValue(ResultElements.data(), ResultElements.size()), E);
   }
+
+  case X86::BI__builtin_ia32_palignr128:
+  case X86::BI__builtin_ia32_palignr256:
+  case X86::BI__builtin_ia32_palignr512: {
+    APValue R;
+    if (!evalShuffleGeneric(Info, E, R, [](unsigned DstIdx, unsigned Shift) {
+          // Default to -1 → zero-fill this destination element
+          unsigned VecIdx = 1;
+          int ElemIdx = -1;
+
+          int Lane = DstIdx / 16;
+          int Offset = DstIdx % 16;
+
+          // Elements come from VecB first, then VecA after the shift boundary
+          unsigned ShiftedIdx = Offset + Shift;
----------------
RKSimon wrote:

```suggestion
          unsigned ShiftedIdx = Offset + (Shift & 0xFF);
```

https://github.com/llvm/llvm-project/pull/162005