x86: Small change to use a total helper function from Macaw

[langston-barrett]

This function has a case marked Bug?:

saw-script/saw-central/src/SAWCentral/X86.hs

Lines 396 to 410 in 04597a9

	callHandler :: Overrides -> CallHandler
	callHandler callMap sym = Macaw.LookupFunctionHandle $ \st mem regs -> do
	case lookupX86Reg X86_IP regs of
	Just (RV ptr) | LLVMPointer base off <- ptr ->
	case (asNat base, BV.asUnsigned <$> asBV off) of
	(Just b, Just o) ->
	case Map.lookup (b,o) callMap of
	Just h -> case h sym of
	Macaw.LookupFunctionHandle f -> f st mem regs
	Nothing ->
	fail ("No over-ride for function: " ++ show (ppPtr ptr))
	_ -> fail ("Non-static call: " ++ show (ppPtr ptr))
	_ -> fail "[Bug?] Failed to obtain the value of the IP register."

Indeed, it is actually an infeasible case. This could be made more obvious by indexing into the register struct with rip from macaw-symbolic. For an example of how this is done, see this GREASE commit.