Refactor the disabling of Thumb-1 LDM/STM generation

Originally I switched the LD/ST optimizer off in TargetMachine as it was previously, but Eric has suggested he'd prefer that it be short-circuited in the pass itself.

No functionality change.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211037 91177308-0d34-0410-b5e6-96231b3b80d8