The mask that compiles to nothing: how HotSpots JIT learned to reason about bits

The part of this that's the most interesting to me is the fact that it is worthwhile for the compiler to expend the effort looking for this optimization opportunity. I would expect (x << 2) & -4 to be a fairly rare pattern, and even then removing the & -4 only saves one or two assembly instructions.

Why is this known bits optimization being done by the JIT rather than the Java compiler?