Force prefetches to check cache and MSHRs immediately prior to issue.

Force prefetches to check cache and MSHRs immediately prior to issue.
This prevents redundant prefetches from being issued, solving the
occasional 'needsExclusive && !blk->isWritable()' assertion failure
in cache_impl.hh that several people have run into.
Eliminates "prefetch_cache_check_push" flag, neither setting of
which really solved the problem.