Roll r123609 back in with two changes that fix test failures with expensive

checks enabled:

1) Use '<' to compare integers in a comparison function rather than '<='.

2) Use the uniqued set DefBlocks rather than Info.DefiningBlocks to initialize
the priority queue.

The speedup of scalarrepl on test-suite + SPEC2000 + SPEC2006 is a bit less, at
just under 16% rather than 17%.

llvm-svn: 123662