[InstCombine] improve efficiency of bool logic; NFC

As noted in issue #59266, the logic reduction could be
beyond the capabilities of an optimizing compiler, and
the code with ternary op is easier to read either way.

llvm/lib/Transforms/InstCombine/InstCombineVectorOps.cpp

@@ -1540,8 +1540,7 @@ static Instruction *foldTruncInsElt(InsertElementInst &InsElt, bool IsBigEndian,
 
   unsigned NumEltsInScalar = ScalarWidth / VecEltWidth;
   Value *X = T;
-  if ((IsBigEndian && IndexC == NumEltsInScalar - 1) ||
-      (!IsBigEndian && IndexC == 0)) {
+  if (IndexC == (IsBigEndian ? NumEltsInScalar - 1 : 0)) {
     // The insert is to the LSB end of the vector (depends on endian).
     // That's all we need.
   } else {