ARM: align loops to 4 bytes on Cortex-M3 and Cortex-M4.

The Technical Reference Manuals for these two CPUs state that branching
to an unaligned 32-bit instruction incurs an extra pipeline reload
penalty. That's bad.

This also enables the optimization at -Os since it costs on average one
byte per loop in return for 1 cycle per iteration, which is pretty good
going.

llvm-svn: 342127