[debug-info] Followup to e0374fb2, avoid changing codegen

As discussed on D140404, calling GetValueInMiddleOfBlock can force PHI
nodes to be created, but the patch was only supposed to fetch a Value if it
wasn't going to generate any PHIs. Fix this by using GetValueAtEndOfBlock
instead of the Middle variant, which was the original intention given the
check of HasValueForBlock.

We don't need to consider the "middle of block" scenario as we're only
dealing with blocks where there are no value definitions.