GenerateMaxInstructionBytes doesn't properly follow nested subrules

[vbmacher]

Example:

root instruction;

instruction = \"a\": 0x00 other | \"b\": 0x10;
other = 0x11 ref16;
ref16 = ref16: ref16(16);

%%

"%s\" = instruction;

generates maxInstructionBytes = 3 instead of 4.