[RISCV] Move NodeExtensionHelper assert to getOrCreateExtendedOp. NFC

Move the narrow types assert from the ZERO_EXTEND/SIGN_EXTEND case in
fillUpExtensionSupport to getOrCreateExtendedOp so we check the other nodes
too.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -13490,6 +13490,7 @@ struct NodeExtensionHelper {
     MVT NarrowVT = getNarrowType(Root, *SupportsExt);
 
     SDValue Source = getSource();
+    assert(Subtarget.getTargetLowering()->isTypeLegal(Source.getValueType()));
     if (Source.getValueType() == NarrowVT)
       return Source;
 
@@ -13657,9 +13658,6 @@ struct NodeExtensionHelper {
       unsigned ScalarBits = VT.getScalarSizeInBits();
       unsigned NarrowScalarBits = NarrowVT.getScalarSizeInBits();
 
-      assert(
-          Subtarget.getTargetLowering()->isTypeLegal(NarrowElt.getValueType()));
-
       // Ensure the extension's semantic is equivalent to rvv vzext or vsext.
       if (ScalarBits != NarrowScalarBits * 2)
         break;