ARM: don't size-reduce STMs using the LR register.

The only Thumb-1 multi-store capable of using LR is the PUSH instruction, which
translates to STMDB, so we shouldn't convert STMIAs.

Patch by Sergey Dmitrouk.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217498 91177308-0d34-0410-b5e6-96231b3b80d8