[Wasm RyuJIT] Load PEP Only Once for JIT Helper Calls

Currently, we duplicate the instruction sequence for loading the PEP for a helper. Ideally, we could anticipate the need for a local to hold the PEP and re-use the loaded PEP value. This may require anticipating everywhere we might emit a JIT helper call in codegen earlier on in `lower`, which could be a bit involved.