Don't require pseudo-instructions to carry encoding information.

For now this is distinct from isCodeGenOnly, as code-gen-only
instructions can (and often do) still have encoding information
associated with them. Once we've migrated all of them over to true
pseudo-instructions that are lowered to real instructions prior to
the printer/emitter, we can remove isCodeGenOnly and just use isPseudo.

llvm-svn: 134539