We developed a new fuzzer, Alive-mutate, that randomly alters an LLVM module and then invokes the Alive2 translation validation tool to see if the mutated module is optimized correctly. Alive-mutate achieves high throughput by avoiding the creation of invalid IR and also by running in the same address space as Alive2, keeping OS-related overhead out of our fuzzing loop. We support 9 different kinds of mutation and have used Alive-mutate to find 23 LLVM bugs including 10 miscompilation bugs in the AArch64 backend and 5 crashes in the instruction combiner.