added functionality to llvm.mc.Instr to print itself

uses binding of printInst on MCInstPrinter for printing
instruction.

also fixed a bug in llvm.mc.Disassembler.decode: idx should
start at zero, and we should pass code.getBase() + idx to
getInstruction. idx must start at zero because the while loop
compares it against code.getExtent() which returns only the
length of bs.