Get allocation orders from RegisterClassInfo when possible.

Only target-dependent hints require callbacks. The RCI allocation order
has CSR aliases last according to their order of appearance in the
getCalleeSavedRegs list. This can depend on the calling convention.

This way, AllocationOrder::next doesn't have to check for reserved
registers, and CSRs are always allocated last, even with weird calling
conventions.

llvm-svn: 132690