[lldb] Drop RegisterInfoInterface::GetDynamicRegisterInfo

"Dynamic register info" is a very overloaded term, and this particular
instance of it was only used for passing the information about the
"orig_[re]ax" pseudo-register on x86 through some generic code. Since
both sides of the code are x86-specific, I have replaced this with a
more direct route.

Differential Revision: https://reviews.llvm.org/D147045