ARM stm/ldm instructions require more than one register in the register list.

ARM stm/ldm instructions require more than one register in the register list.
Otherwise, a plain str/ldr should be used instead. Make sure we account for
that in prologue/epilogue code generation.
rdar://8745460

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121391 91177308-0d34-0410-b5e6-96231b3b80d8