[GVN] Common code for local and non-local load availability [NFCI]

The attached patch removes all of the block local code for performing X-load forwarding by reusing the code used in the non-local case.

The motivation here is to remove duplication and in the process increase our test coverage of some fairly tricky code. I have some upcoming changes I'll be proposing in this area and wanted to have the code cleaned up a bit first.

Note: The review for this mostly happened in email which didn't make it to phabricator on the 258882 commit thread.

Differential Revision: http://reviews.llvm.org/D16608

llvm-svn: 260711