Add flags in instr to indicate if an instruction read/write memory.

_From [qin.zhao@gmail.com](https://code.google.com/u/qin.zhao@gmail.com/) on April 19, 2010 15:41:02_

Add flag in instr to indicate if an instruction reads or writes memory.
This is to improve the performance of instr_reads_memory and
instr_writes_memory.
We should have flag to show read/write and unknown, so functions only need
iterate the operands if it is unknown.

_Original issue: http://code.google.com/p/dynamorio/issues/detail?id=293_