and simplify code that was fallout from the separation of memcpyopt and gvn.

llvm-svn: 50034

GVN and into its own pass.

llvm-svn: 49419

llvm-svn: 48938

llvm-svn: 48732

llvm-svn: 48731

This fires dozens of times across spec and multisource, but I don't know
if it actually speeds stuff up.  Hopefully the testers will show something
nice :)

llvm-svn: 48680