Target RegisterInfo: devirtualize TargetFrameLowering

Summary:
The target frame lowering's concrete type is always known in RegisterInfo, yet it's only sometimes devirtualized through a static_cast. This change adds an auto-generated static function <Target>GenRegisterInfo::getFrameLowering(const MachineFunction &MF) which does this devirtualization, and uses this function in all targets which can.

This change was suggested by sunfish in D11070 for WebAssembly, I figure that I may as well improve the other targets while I'm here.

Subscribers: sunfish, ted, llvm-commits, jfb

Differential Revision: http://reviews.llvm.org/D11093

llvm-svn: 241921