It is, but there's a loophole: the prosecution can argue that the police inevitably would have discovered the evidence eventually. It's fairly easy to claim for something like a murder because hiding a body permanently tends to be fairly difficult, and arguing that the murderer would have successfully hidden the body is very close to an admission of guilt.

Edit: if the police interrupt and prevent a murder, the testimony of the murder victim is also admissible. I don't think the argument "if the police hadn't illegally searched the area, the victim would be dead and unable to testify" works.