Function dtrace_difo_validate

* Validate a DTrace DIF object by checking the IR instructions. The following * rules are currently enforced by dtrace_difo_validate(): * * 1. Each instruction must have a valid opcode * 2. Each register, string, variable, or subroutine reference must be valid * 3. No instruction can modify register %r0 (must be zero) * 4. All instruction reserved bits must be set to zero * 5. The last ins