[ARM] Don't try to create "push {r12, lr}" in Thumb1 at -Oz.

It's a little tricky to make this issue show up because
prologue/epilogue emission normally likes to push at least two
registers... but it doesn't when lr is force-spilled due to function
length.  Not sure if that really makes sense, but I decided not to touch
it for now.

Differential Revision: https://reviews.llvm.org/D59385

llvm-svn: 357436