[GVN] Split AvailableValueInBlock into two parts [NFC]

AvailableValue is the part that represents the potential rematerialization.  AvailableValueInBlock is simply a pair of an AvailableValue and a BB which we might materialize it in.

This is motivated by http://reviews.llvm.org/D16608.  The intent is that we'll have a single function which handles the local case which both local and non-local will use to identify available values.  Once that's done, the local case can rematerialize at the use site and the non-local case can do the SSA construction as it does currently.

llvm-svn: 258882