[TargetLowering] use stable_sort to avoid nondeterminism

After 330fa7d2 we were seeing nondeterministic failures of
llvm/test/CodeGen/ARM/thumb-big-stack.ll, with different code being
generated in different runs.

Switching sort -> stable_sort fixes this.
It looks like the old algorithm picked the first best option, and using
stable_sort restores that behavior.