[ARM] Transform LDMs into writeback form to save code size

If we have an LDM that uses only low registers and doesn't write to its base register:

  ldm.w r0, {r1, r2, r3}

And that base register is dead after the LDM, then we can convert it to writeback form and use a narrow encoding:

  ldm.n r0!, {r1, r2, r3}

Obviously, this introduces a new register write and so can cause WAW hazards, so I've enabled it only in minsize mode. This is a code size trick that ARM Compiler 5 ("armcc") does that we don't.

llvm-svn: 272000