X86: Correct IR type used for sincos_stret return value (#166240)

Match the IR type that clang uses here: https://godbolt.org/z/KzbodEcxh

This was manually selecting the IR legal type. Instead just set the
flag to ensure legal types.

llvm/lib/Target/X86/X86ISelLowering.cpp

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 2970cf4..b97b508 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -33034,12 +33034,13 @@
       DAG.getExternalSymbol(LibcallName, TLI.getPointerTy(DAG.getDataLayout()));
 
   Type *RetTy = isF64 ? (Type *)StructType::get(ArgTy, ArgTy)
-                      : (Type *)FixedVectorType::get(ArgTy, 4);
+                      : (Type *)FixedVectorType::get(ArgTy, 2);
 
   TargetLowering::CallLoweringInfo CLI(DAG);
   CLI.setDebugLoc(dl)
       .setChain(DAG.getEntryNode())
-      .setLibCallee(CallingConv::C, RetTy, Callee, std::move(Args));
+      .setLibCallee(CallingConv::C, RetTy, Callee, std::move(Args))
+      .setIsPostTypeLegalization();
 
   std::pair<SDValue, SDValue> CallResult = TLI.LowerCallTo(CLI);