4.9.1: Prohibit the ret opcode in the code array of >= 51.0 class files

JVMS 4.9.1 "Static Constraints" says "If the class file version number is 51.0 or above, then neither the jsr opcode or the jsr_w opcode may appear in the code array." The ret opcode should be added to this constraint because HotSpot issues a VerifyError with the following message when it encounters a ret opcode in a class file with version 51.0 or above:

exception: Bad instruction: a9
Exception Details:
  Location:
    primArray.castToByteArray([I)[B @1: ret

More specifically, in a >=51.0 class file, HotSpot treats jsr (a8), jsr_w (c9), and ret (a9) the same as the three reserved opcodes (ca, fe, ff) and the numerous undocumented opcodes (cb-fd): there are no type-checking rules for any of these opcodes, so they are unrecognized and cause VerifyErrors. This is a valid way to "implement" the first bullet of 4.9.1.

The constraint about jsr/jsr_w/ret should mirror the constraint about reserved/undocumented opcodes, as follows: "If the class file version number is 51.0 or above, then instances of instructions using the jsr, jsr_w, or ret opcodes must not appear in the code array."