target-mips: copy insn_flags in DisasContext

Copy insn_flags in DisasContext to avoid passing a CPUMIPSState pointer
to subroutines, as suggested by Richard Henderson. Change subroutines to
use this new field and remove the first argument.
Reviewed-by: NRichard Henderson <rth@twiddle.net>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>