[clang] [Clang] Allow AVX/AVX512 subvector shuffles in constexpr (PR #168700)

[Simon Pilgrim via cfe-commits]

================
@@ -13517,6 +13518,56 @@ bool VectorExprEvaluator::VisitCallExpr(const CallExpr *E) {
     return Success(APValue(ResultElements.data(), ResultElements.size()), E);
   }
 
+  case X86::BI__builtin_ia32_shuf_f32x4_256:
+  case X86::BI__builtin_ia32_shuf_i32x4_256:
+  case X86::BI__builtin_ia32_shuf_f64x2_256:
+  case X86::BI__builtin_ia32_shuf_i64x2_256:
+  case X86::BI__builtin_ia32_shuf_f32x4:
+  case X86::BI__builtin_ia32_shuf_i32x4:
+  case X86::BI__builtin_ia32_shuf_f64x2:
+  case X86::BI__builtin_ia32_shuf_i64x2: {
+    APValue SourceA, SourceB;
+    if (!EvaluateAsRValue(Info, E->getArg(0), SourceA) ||
+        !EvaluateAsRValue(Info, E->getArg(1), SourceB))
+      return false;
+
+    APSInt Imm;
+    if (!EvaluateInteger(E->getArg(2), Imm, Info))
+      return false;
+
+    // Destination and sources A, B all have the same type.
+    unsigned NumElems = SourceA.getVectorLength();
+    const VectorType *VT = E->getArg(0)->getType()->castAs<VectorType>();
+    QualType ElemQT = VT->getElementType();
+    unsigned ElemBits = Info.Ctx.getTypeSize(ElemQT);
+    constexpr unsigned LaneBits = 128u;
----------------
RKSimon wrote:

```suggestion
    unsigned LaneBits = 128u;
```

https://github.com/llvm/llvm-project/pull/168700