ARM: sort register lists by encoding in push/pop instructions.

ARM: sort register lists by encoding in push/pop instructions.

For example we were producing

    push {r8, r10, r11, r4, r5, r7, lr}

This is misleading (r4, r5 and r7 are actually pushed before the rest), and
other components (stack folding recently) often forget to deal with the extra
complexity coming from the different order, leading to miscompiles. Finally, we
warn about our own code in -no-integrated-as mode without this, which is really
not a good idea.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286866 91177308-0d34-0410-b5e6-96231b3b80d8