[InstCombine] rename SimplifyDivRemOfSelect to be clearer, add comments, simplify code; NFCI

There's at least one bug here - this code can fail with vector types (PR34856).
It's also being called for FREM; I'm still trying to understand how that is valid.

llvm-svn: 315127
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index 6b7bcd9..46f8611 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -277,7 +277,7 @@
   Instruction *visitURem(BinaryOperator &I);
   Instruction *visitSRem(BinaryOperator &I);
   Instruction *visitFRem(BinaryOperator &I);
-  bool SimplifyDivRemOfSelect(BinaryOperator &I);
+  bool simplifyDivRemOfSelectWithZeroOp(BinaryOperator &I);
   Instruction *commonRemTransforms(BinaryOperator &I);
   Instruction *commonIRemTransforms(BinaryOperator &I);
   Instruction *commonDivTransforms(BinaryOperator &I);