[MIPS] RFA: Use new rtl iterators in mips_sim_wait_regs_1

[Matthew Fortune]

> gcc/
> 	* config/mips/mips.c (mips_sim_insn): Update comment.
> 	(mips_sim_wait_regs_2): Delete.
> 	(mips_sim_wait_regs_1): Use FOR_EACH_SUBRTX_VAR.

OK