Don't try to be clever, just preserve the target's allocation order.

The order of registers returned by getCalleeSavedRegs is used to lay out
the fixed stack slots for CSRs. Some targets like their CSRs used from
one end, and some targets want them used from the other end.

When computing an allocation order, simply preserve the relative
ordering of CSRs that the target specifies in its allocation order.
Reordering CSRs would break some targets, ARM in particular.

We still place volatiles before the CSRs, providing slightly better
results with different calling conventions.

llvm-svn: 132680