optimize definedness propagation by identifying memrefs in module data sections

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on March 07, 2013 11:08:25_

Idea from Qin: for absolute memrefs on x86 and rip-rel on x64 we should be
able to tell at instrumentation time whether they are in a module's data
section, which is always defined. We can then skip instrumentation when
combining them with other globals or with immediates.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=1164_