[RISCV][ISel] Attempt to fix UBSan error

Explicitly check an SDValue with the invalid SDValue.

UBSan reports:
runtime error: load of value 36, which is not a valid value for type
'bool'

https://lab.llvm.org/buildbot/#/builders/85/builds/11231

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -8491,7 +8491,9 @@ struct NodeExtensionHelper {
   }
 
   /// Check if this operand is compatible with the given vector length \p VL.
-  bool isVLCompatible(SDValue VL) const { return this->VL && this->VL == VL; }
+  bool isVLCompatible(SDValue VL) const {
+    return this->VL != SDValue() && this->VL == VL;
+  }
 
   /// Check if this operand is compatible with the given \p Mask.
   bool isMaskCompatible(SDValue Mask) const {