S12Z opcodes: Fix bug disassembling certain shift instructions.

Shift and rotate instructions when the number of bit positions
was an immediate value greater than 1 were incorrectly disassembled.
This change fixes that problem and extends the test to check for
it.

gas/ChangeLog:

  testsuite/gas/s12z/shift.s: Add new test case.
  testsuite/gas/s12z/shift.d: Add expected result.

opcodes/ChangeLog:

  s12z-dis.c (print_insn_shift) [SB_REG_REG_N]: Enter special case
  if the postbyte matches the appropriate pattern.