[LowerSwitch] Fixed faulty PHI nodes
Fixed two cases of where PHI nodes need to be updated by lowerswitch.
When lowerswitch find out that the switch default branch is not
reachable it remove the old default and replace it with the most
popular block from the cases, but it forget to update the PHI
nodes in the default block.
The PHI nodes also need to be updated when the switch is replaced
with a single branch.
Reviewers: hans, reames, arsenm
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D47203