mips: fix corner case for INS instruction

This change fixes corner case for INS instruction when lsb = 0.
The test in none/tests/mips32/MIPS32int.c will be extended to include
additional test cases that trigger this condition.


git-svn-id: svn://svn.valgrind.org/vex/trunk@2715 8f6e269a-dfd6-0310-a8e1-e2731360e62c