Add a simple little improvement to the local spiller to keep track of stores

and delete them if they turn out to be dead.  This is a useful little hack
that even speeds up some programs.  For example, it speeds up Ptrdist/ks
from 17.53s to 15.59s, and 188.ammp from 149s to 146s.

This also speeds up llc :)

llvm-svn: 16630