Both MCRegisterInfo and MCSubtargetInfo are non-null at every callsite that matters (only nullable in unit tests like `llvm/unittests/CodeGen/MFCommon.inc`), mirroring the recent `const MCAsmInfo &` cleanup. * TargetMachine::getMCRegisterInfo and getMCSubtargetInfo return references. * MCContext's constructor takes const MCRegisterInfo & and const MCSubtargetInfo &.