[RISCV] Reorder insert_subvector and bitcast in getDeinterleaveShiftAndTrunc. (llvm#127258)

Put the bitcast before the insert_subvector. It's more likely the insert
subvector can be combined with other nodes. The insert_subvector is only
needed sometimes, and I'm considering reusing this function which might
require pulling the insert_subvector out.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -4660,10 +4660,10 @@ static SDValue getDeinterleaveShiftAndTrunc(const SDLoc &DL, MVT VT,
   SDValue Res = DAG.getNode(ISD::SRL, DL, WideSrcVT, Src,
                             DAG.getConstant(Shift, DL, WideSrcVT));
   Res = DAG.getNode(ISD::TRUNCATE, DL, ResVT, Res);
-  MVT IntVT = VT.changeVectorElementTypeToInteger();
-  Res = DAG.getNode(ISD::INSERT_SUBVECTOR, DL, IntVT, DAG.getUNDEF(IntVT), Res,
-                    DAG.getVectorIdxConstant(0, DL));
-  return DAG.getBitcast(VT, Res);
+  MVT CastVT = ResVT.changeVectorElementType(VT.getVectorElementType());
+  Res = DAG.getBitcast(CastVT, Res);
+  return DAG.getNode(ISD::INSERT_SUBVECTOR, DL, VT, DAG.getUNDEF(VT), Res,
+                     DAG.getVectorIdxConstant(0, DL));
 }
 
 // Lower the following shuffle to vslidedown.