Perform NULL segment checks upfront in opcode handlers, rather than on every memory access.

This removes the need to pass the segment to readmem*l()/writemem*l().

src/x86_ops_mmx_shift.h

@@ -6,6 +6,7 @@
         }                                                               \
         else                                                            \
         {                                                               \
+                SEG_CHECK_READ(cpu_state.ea_seg);                       \
                 shift = readmemb(easeg, cpu_state.eaaddr); if (cpu_state.abrt) return 0;    \
                 CLOCK_CYCLES(2);                                        \
         }