Cache: Split invalidateBlk up to seperate block vs. tags

Cache: Split invalidateBlk up to seperate block vs. tags

This seperates the functionality to clear the state in a block into
blk.hh and the functionality to udpate the tag information into the
tags.  This gets rid of the case where calling invalidateBlk on an
already-invalid block does something different than calling it on a
valid block, which was confusing.