Removed arithmetic opcodes with immediate operand

The difference in performance between immediate operands and K operands
does not seem to justify all those extra opcodes. We only keep OP_ADDI,
due to its ubiquity and because the difference is a little more relevant.
(Later, OP_SUBI will be implemented by OP_ADDI, negating the constant.)

lopnames.h

@@ -30,12 +30,6 @@ static const char *const opnames[] = {
   "NEWTABLE",
   "SELF",
   "ADDI",
-  "SUBI",
-  "MULI",
-  "MODI",
-  "POWI",
-  "DIVI",
-  "IDIVI",
   "ADDK",
   "SUBK",
   "MULK",