Don't use additional arguments for dss and friends to satisfy DSS_Form,

Don't use additional arguments for dss and friends to satisfy DSS_Form,
when let can do the same thing. Keep the 64bit variants as codegen-only.
While they have a different register class, the encoding is the same for
32bit and 64bit mode. Having both present would otherwise confuse the
disassembler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214636 91177308-0d34-0410-b5e6-96231b3b80d8