Fix a bug in sdisel switch lowering code. When it updates the phi nodes in switch...

Fix a bug in sdisel switch lowering code. When it updates the phi nodes in switch successor blocks, it can introduce multiple phi operands of the same value from different blocks (and may not be on the predecessor list).

This can be seen on CodeGen/Generic/2006-09-06-SwitchLowering.ll. But it's not known to cause any real regression (but I have added an assertion for it now).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82214 91177308-0d34-0410-b5e6-96231b3b80d8