If all the bit positions are not specified; do not decode the instructions.

If all the bit positions are not specified; do not decode the instructions.
We are bound to fail!  For proper disassembly, the well-known encoding bits
of the instruction must be fully specified.

This also removes pseudo instructions from considerations of disassembly,
which is a better design and less fragile than the name matchings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@100899 91177308-0d34-0410-b5e6-96231b3b80d8