[LLDB] Pass const RegisterInfo& to RegisterValue::SetValueFromData

Familiar story, callers are either checking upfront that the pointer
wasn't null or not checking at all. SetValueFromData itself didn't
check either.

So make the parameter a ref and fixup the few places where a nullptr
check seems needed.

Depends on D135668

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D135670