LowerSwitch: Fix producing invalid IR on unreachable code

If a switch was in an unreachable block that branched
to a block with a phi, it would leave phis with missing
predecessors.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301064 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed