Make JIT inline cast_for_multi_block_arg correctly

(Bug originally found in rbx-1.3.3)

JIT was not inlining arguments correctly when given more than one
argument, and would end up yielding cUndef (Object:0x22) instead.

See https://gist.github.com/sshao/7eecc87458d3e55d7f5d
(run on rbx-1.3.3) for examples.