Remove MethodProtos/MethodBodies and allocation_order_begin/end.

Targets that need to change the default allocation order should use the
AltOrders mechanism instead. See the X86 and ARM targets for examples.

The allocation_order_begin() and allocation_order_end() methods have been
replaced with getRawAllocationOrder(), and there is further support
functions in RegisterClassInfo.

It is no longer possible to insert arbitrary code into generated
register classes. This is a feature.

llvm-svn: 133332