[SimplifyCFG] fix accessing deleted PHINodes in switch-to-table conversion.

When we have a covered lookup table, make sure we don't delete PHINodes that
are cached in PHIs.

rdar://17887153


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214642 91177308-0d34-0410-b5e6-96231b3b80d8