[jit] Decompose long immediate shifts

[jit] Decompose long immediate shifts

We decompose them as part of the cprop pass to handle more immediates and to further propagate copies.

The native backend of some architectures (for example x86) may support long shifts with specialized instructions. Even there, this still makes immediate shifts 2 times faster because long ops are poorly handled by the jit.

On arm, where we were emulating, this makes long immediate shifts 3-5 times faster.