ARM: don't rely on push/pop reglists being in order when folding SP adjust.

It would be a very nice invariant to rely on, but unfortunately it doesn't
necessarily hold (and the causes of mis-sorted reglists appear to be quite
varied) so to be robust the frame lowering code can't assume that the first
register in the list is also the first one that actually gets pushed.

Should fix an issue where we were turning something like:

    push {r8, r4, r7, lr}
    sub sp, #24

into nonsense like:

    push {r2, r3, r4, r5, r6, r7, r8, r4, r7, lr}

llvm-svn: 285232