When adding blocks to the list of those which no longer have any CFG

When adding blocks to the list of those which no longer have any CFG
conflicts, we should only be adding the first block of the chain to the
list, lest we try to merge into the middle of that chain. Most of the
places we were doing this we already happened to be looking at the first
block, but there is no reason to assume that, and in some cases it was
clearly wrong.

I've added a couple of tests here. One already worked, but I like having
an explicit test for it. The other is reduced from a test case Duncan
reduced for me and used to crash. Now it is handled correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145119 91177308-0d34-0410-b5e6-96231b3b80d8