When save/restoring CR at prolog/epilog, in a large

When save/restoring CR at prolog/epilog, in a large
stack frame, the prolog/epilog code was using the same
register for the copy of CR and the address of the save slot.  Oops.
This is fixed here for Darwin, sort of, by reserving R2 for this case.
A better way would be to do the store before the decrement of SP,
which is safe on Darwin due to the red zone.

SVR4 probably has the same problem, but I don't know how to fix it;
there is no red zone and R2 is already used for something else.
I'm going to leave it to someone interested in that target.

Better still would be to rewrite the CR-saving code completely;
spilling each CR subregister individually is horrible code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@96015 91177308-0d34-0410-b5e6-96231b3b80d8