Fix Bug 327284. The condition code of risbg was not correct. 
This instruction might be used by by gcc for masking out bits,
e.g. code like 
 n &= 3;
  if (n == 0)

might result in 
        risbg   %r4,%r4,62,128+63,0
        je      <target>

The old code set the condition code depending on the operand before
masking. Fix it. This patch also indicates that we need test suite
coverage for risbg and friends.




git-svn-id: svn://svn.valgrind.org/vex/trunk@2798 8f6e269a-dfd6-0310-a8e1-e2731360e62c