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.

llvm/target.py

@@ -225,6 +225,19 @@ class TargetMachine(llvm.Wrapper):
 
             return self._dasm
 
+        @property
+        def inst_printer(self):
+            if not getattr(self, '_mip', False):
+                self._mip = self.target.createMCInstPrinter(
+                                     self.asm_info.getAssemblerDialect(),
+                                     self.asm_info,
+                                     self.instr_info,
+                                     self.reg_info,
+                                     self.subtarget_info
+                                     )
+
+            return self._mip
+
         def is_little_endian(self):
             return self.asm_info.isLittleEndian()