Given an invocation context block and general-purpose register
    index such that 0 <= index < 16, copy the register value to
    gr_copy, for example, index 4 fetches the invocation context
    block IREG[4] value, which represents the contents of %rsi for
    the context. LIB$X86_GET_GR fails if the index represents a
    scratch register whose contents have not been realized.

    Format
      LIB$X86_GET_GR (invo_context, index, gr_copy)