optimizing pattern mode instrumentation code

_From [zhao...@google.com](https://code.google.com/u/106321947286816917100/) on February 06, 2012 15:54:40_

In pattern mode, we insert code before app's memory reference to check if it accesses unaddressable redzone filled with pattern value.
The naive instrumentation is to insert context switch code and checks before every app instructions.
However, there are many optimization opportunities, including:
1. being smart on context switch to avoid unnecessary register save/restore
2. remove redundant checks of multiple instructions accessing the same memory address.
3. move checks around to avoid context switches.
...

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=776_