[VPlan] Inline WidenSelect::isInvariantCond (NFC) (llvm#166742)

VPWidenSelectRecipe::isInvariantCond has a sole use: inline it in the
use-site, as it is not meant to be used standalone.

Author: artagnon

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -1762,15 +1762,11 @@ struct LLVM_ABI_FOR_TEST VPWidenSelectRecipe : public VPRecipeWithIRFlags,
     return getOperand(0);
   }
 
-  bool isInvariantCond() const {
-    return getCond()->isDefinedOutsideLoopRegions();
-  }
-
   /// Returns true if the recipe only uses the first lane of operand \p Op.
   bool usesFirstLaneOnly(const VPValue *Op) const override {
     assert(is_contained(operands(), Op) &&
            "Op must be an operand of the recipe");
-    return Op == getCond() && isInvariantCond();
+    return Op == getCond() && Op->isDefinedOutsideLoopRegions();
   }
 };