keep movzx 1-to-4 partial-unaddr and 2-to-4 partial-undef on fastpath

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 17:58:14_

PR 503782

movzx 1-to-4 load addressability should check just the referenced 2 bits.

movzx 2-to-4 should propagate instead of bailing to check_definedness.

neither should be difficult to implement and should help twolf, vpr, and
other benchmarks.

PR 503782: keep movzx 1-to-4 partial-unaddr and 2-to-4 partial-undef
on fastpath
- movzx 1-to-4 load: check just the referenced 2 bits for addressability
- movzx 2-to-4: propagate instead of check_definedness
twolf test slowpath reductions:
tot:
195 movzx: 209291
200 movsx: 888302
1-to-4:
195 movzx: 184074
200 movsx: 515750
- 2-to-4:
195 movzx: 32

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=172_