Some minor optimizations.

Calculating -------------------------------------
                           ours       trunk
          vm2_regexp     8.479M      8.346M i/s -      6.000M times in 0.707612s 0.718916s
   vm2_regexp_invert     8.605M      8.350M i/s -      6.000M times in 0.697298s 0.718576s

Comparison:
                       vm2_regexp
                ours:   8479223.3 i/s
               trunk:   8345893.8 i/s - 1.02x  slower

                vm2_regexp_invert
                ours:   8604647.4 i/s
               trunk:   8349852.8 i/s - 1.03x  slower

Calculating -------------------------------------
                           ours+jit   trunk+jit
Optcarrot Lan_Master.nes     68.603      64.167 fps

Comparison:
             Optcarrot Lan_Master.nes
                ours+jit:        68.6 fps
               trunk+jit:        64.2 fps - 1.07x  slower