[clang] [clang][X86] Allow VALIGND/Q element-shift intrinsics in constexpr evaluation (PR #168206)

[Simon Pilgrim via cfe-commits]

================
@@ -13551,6 +13551,30 @@ bool VectorExprEvaluator::VisitCallExpr(const CallExpr *E) {
       return false;
     return Success(R, E);
   }
+  case X86::BI__builtin_ia32_alignd128:
+  case X86::BI__builtin_ia32_alignd256:
+  case X86::BI__builtin_ia32_alignd512:
+  case X86::BI__builtin_ia32_alignq128:
+  case X86::BI__builtin_ia32_alignq256:
+  case X86::BI__builtin_ia32_alignq512: {
+    APValue R;
+    const unsigned NumElts =
+        E->getType()->castAs<VectorType>()->getNumElements();
+    if (!evalShuffleGeneric(
+            Info, E, R, [NumElts](unsigned DstIdx, unsigned Shift) {
+              unsigned Imm = Shift & 0xFF;
+              unsigned EffectiveShift = Imm & (NumElts - 1);
+              unsigned SourcePos = DstIdx + EffectiveShift;
+              unsigned VecIdx = SourcePos < NumElts ? 1 : 0;
+              unsigned ElemIdx =
+                  SourcePos < NumElts ? SourcePos : SourcePos - NumElts;
----------------
RKSimon wrote:

```suggestion
                  unsigned ElemIdx = SourcePos & (NumElems - 1);
```

https://github.com/llvm/llvm-project/pull/168206