ARM: When spilling extra registers for alignment, prefer low registers on all Thumb...

ARM: When spilling extra registers for alignment, prefer low registers on all Thumb targets.

This makes it more likely that we can use the 16-bit push and pop instructions
on Thumb-2, saving around 4 bytes per function.

Differential Revision: http://reviews.llvm.org/D9165

llvm-svn: 235637