regcache: Fix call to address lookup function

It was previously using JIT_R0 which apparently ended up spilled
by error to the register cache at some point.
lightrec: Use only callee-saved registers in the address lookup

This avoids spilling registers to the cache when not necessary.
emitter: Optimize store of next_pc from compiled code
emitter: Pre-load the registers used in the delay slot

Otherwise they will be loaded from the register cache only if the branch
is taken.
lightrec: Free unused memory after recompiling blocks