eflags propagation in fastpath to subdword requires mapping 4-to-1 or 4-to-2

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on May 15, 2011 13:37:46_

b/c I'm using a full shadow byte for eflags and for efficiency I'm writing
undef bits to all parts of it, when propagating eflags in fastpath, if dst
is sub-dword we'd lose undef bits w/ a straight-up propagation at offs 0.
could special-case w/ cmp to 0 and write (won't be unaddr).

propagation happens not just on instrs that read nothing but eflags (setcc,
lahf, pushfw (pushfw is already on slowpath)) but also (xref PR 425622)
things like adc and sbb.

for now doing check_eflags_defined. this issue covers propagating when undef.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=402_