Automerge: [DAG] Always use stack to promote bitcast when the source is vector (#151065)

The optimization introduced by #125637 tried to avoid using stacks to
promote bitcast with vector result type. However, it wouldn't be correct
if the input type is vector. This patch limits that optimizations to
only scalar to vector bitcasts.

Author: mshockwave

llvm/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp

@@ -2217,8 +2217,9 @@ SDValue DAGTypeLegalizer::PromoteIntOp_BITCAST(SDNode *N) {
 
   switch (getTypeAction(InVT)) {
   case TargetLowering::TypePromoteInteger: {
-    // TODO: Handle big endian
-    if (OutVT.isVector() && DAG.getDataLayout().isLittleEndian()) {
+    // TODO: Handle big endian & vector input type.
+    if (OutVT.isVector() && !InVT.isVector() &&
+        DAG.getDataLayout().isLittleEndian()) {
       EVT EltVT = OutVT.getVectorElementType();
       TypeSize EltSize = EltVT.getSizeInBits();
       TypeSize NInSize = NInVT.getSizeInBits();