[NFC][InstructionSimplify] Add a warning about not simplifying to not def-reachable

See
https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20200824/824235.html
and
https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20200824/824967.html

InstSimply is not allowed to perform simplifications to instructions
that are not def-reachable from the original instruction.

Author: LebedevRI

llvm/include/llvm/Analysis/InstructionSimplify.h

@@ -26,6 +26,10 @@
 // same call context of that function (and not split between caller and callee
 // contexts of a directly recursive call, for example).
 //
+// Additionally, these routines can't simplify to the instructions that are not
+// def-reachable, meaning we can't just scan the basic block for instructions
+// to simplify to.
+//
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_ANALYSIS_INSTRUCTIONSIMPLIFY_H

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -4403,6 +4403,10 @@ Value *llvm::SimplifyExtractElementInst(Value *Vec, Value *Idx,
 
 /// See if we can fold the given phi. If not, returns null.
 static Value *SimplifyPHINode(PHINode *PN, const SimplifyQuery &Q) {
+  // WARNING: no matter how worthwhile it may seem, we can not perform PHI CSE
+  //          here, because the PHI we may succeed simplifying to was not
+  //          def-reachable from the original PHI!
+
   // If all of the PHI's incoming values are the same then replace the PHI node
   // with the common value.
   Value *CommonValue = nullptr;