[InstSimplify] Respect CanUseUndef in more places

Similar to what we do in IIQ, add an isUndefValue() helper that
checks for undef values while respective CanUseUndef. This makes
it much easier to search for places that don't respect the flag
yet.

Author: nikic

llvm/include/llvm/Analysis/InstructionSimplify.h

@@ -123,6 +123,14 @@ struct SimplifyQuery {
     Copy.CanUseUndef = false;
     return Copy;
   }
+
+  /// If CanUseUndef is true, returns whether \p V is undef.
+  /// Otherwise always return false.
+  bool isUndefValue(Value *V) const {
+    if (!CanUseUndef)
+      return false;
+    return isa<UndefValue>(V);
+  }
 };
 
 // NOTE: the explicit multiple argument versions of these functions are