[SwitchLowering] Fix the "fixPhis" function.

Switch statements may have more than one incoming edge into the same BB if they
all have the same value. When the switch statement is converted these incoming
edges are now coming from multiple BBs. Updating all incoming values to be from
a single BB is incorrect and would generate invalid LLVM IR.

The fix is to only update the first occurrence of an incoming value. Switch
lowering will perform subsequent calls to this helper function for each incoming
edge with a new basic block - updating all edges in the process.

This fixes rdar://problem/18916275.

llvm-svn: 221627