Optimise away bytecode dispatch.

[vext01]

Needs ykjit/yk#2119

yk before

            Mean        Std.Dev.    Min         Median      Max
real        1.504       0.003       1.500       1.505       1.507
user        1.510       0.003       1.506       1.509       1.513
sys         0.008       0.004       0.000       0.008       0.012

yk after

            Mean        Std.Dev.    Min         Median      Max
real        0.166       0.001       0.165       0.166       0.167
user        0.171       0.004       0.166       0.172       0.177
sys         0.006       0.004       0.000       0.004       0.012

upstream baseline

            Mean        Std.Dev.    Min         Median      Max
real        0.247       0.001       0.244       0.247       0.249
user        0.247       0.001       0.244       0.247       0.248
sys         0.000       0.000       0.000       0.000       0.000

[ltratt]

Cant argue with that!