ARM: convert ORR instructions to ADD where possible on Thumb.

Thumb has more 16-bit encoding space dedicated to ADD than ORR, allowing both a
3-address encoding and a wider range of immediates. So, particularly when
optimizing for code size (but it doesn't make things worse elsewhere) it's
beneficial to select an OR operation to an ADD if we know overflow won't occur.

This is made even better by LLVM's penchant for putting operations in canonical
form by converting the other way.

llvm-svn: 335119