In Thumb1, materialize a move between low registers as a `movs`, if CPSR isn't live.

Summary: Previously, it had always been materialized as a push/pop sequence.

Reviewers: labrinea, jroelofs

Reviewed By: jroelofs

Subscribers: llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D30648

llvm-svn: 297134