propagate non-word-sized mem2mem shadow values in fastpath

_From [bruen...@google.com](https://code.google.com/u/109494838902877177630/) on January 03, 2011 19:12:39_

issue #236 added 1) propagation of word-sized

this issue covers:

2) add propagation for non-word-sized by grabbing a 4th scratch reg

3) get pop-mem on fastpath
/\* XXX issue #236: to support mem2mem here we need to update
\* instrument_fastpath to treat the load as the primary for
\* pushpop-ness (normally mem2mem treats the load as secondary and
\* the store as primary)
*/

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=262_