spec2k6 gcc very slow b/c memset not checking in-heap flag

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on December 13, 2010 09:52:44_

my in-heap tls flag check is only enabled for bbs encountered while in-heap: so if memset is called earlier by a non-heap routine, memset's instrumentation won't check the flag, and we can spend a lot of time in the slowpath. this is exactly what happens on spec2k6 gcc.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=234_