[InstCombine] add helper function for select-of-bools folds; NFC

This set of folds keeps growing, and it contains
bugs like issue llvm#58552, so make it easier to
spot those via backtrace.

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineInternal.h

@@ -733,6 +733,7 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
   Instruction *foldICmpBitCast(ICmpInst &Cmp);
 
   // Helpers of visitSelectInst().
+  Instruction *foldSelectOfBools(SelectInst &SI);
   Instruction *foldSelectExtConst(SelectInst &Sel);
   Instruction *foldSelectOpOp(SelectInst &SI, Instruction *TI, Instruction *FI);
   Instruction *foldSelectIntoOp(SelectInst &SI, Value *, Value *);