target-mips: fix translation of MT instructions

The translation of dmt/emt/dvpe/evpe was doing the moral equivalent of:

  int x;
  ...		/* no initialization of x */
  x = f (x);

which confused later bits of TCG rather badly, leading to crashes.

Fix the helpers to only return results (those instructions have no
inputs), and fix the translation code accordingly.

Signed-off-by: Nathan Froyd <froydnj@codesourcery.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
