Don't fold insufficiently aligned ldr/str into ldm/stm instructions.

An unaligned ldr causes a trap, and is then emulated by the kernel with
awesome performance. The darwin kernel does not emulate unaligned ldm/stm
Thumb2 instructions, so don't generate them.

This fixes the miscompilation of Multisource/Applications/JM/lencod for Thumb2.

Generating unaligned ldr/str pairs from a 16-bit aligned memcpy is probably
also a bad idea, but that is beyond the scope of this patch.

llvm-svn: 93393