Mark ARM pseudo-instructions as isPseudo.

Mark ARM pseudo-instructions as isPseudo.

This allows us to remove the (bogus and unneeded) encoding information from
the pseudo-instruction class definitions. All of the pseudos that haven't
been converted yet and still need encoding information instance from the normal
instruction classes and explicitly set isCodeGenOnly, and so are distinct
from this change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134540 91177308-0d34-0410-b5e6-96231b3b80d8