Clean up Thumb load/store multiple definitions.

Clean up Thumb load/store multiple definitions.

There is no non-writeback store multiple instruction in Thumb1, so
don't define one. As a result load multiple is the only instantiation of
the multiclass, so refactor that away entirely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138338 91177308-0d34-0410-b5e6-96231b3b80d8