[PlaceSafepoints] Entry safepoint location doesn't need to be a terminator

Long ago, the poll insertion code assumed that the insertion site was a terminator.  As a result, the entry selection code would split a basic block to ensure it could pass a terminator.  The insertion code was updated quite a while ago - possibly before it ever landed upstream - but the now redundant work was never removed.  

While I'm at it, remove a comment which doesn't apply to the upstreamed code.  

NFC intended.

llvm-svn: 238254